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Kernel development

Brief items

Kernel release status

The current 2.6 development prepatch is 2.6.31-rc1, released on June 24. The 2.6.31 merge window is now closed, and the stabilization phase has begun. As always, see the long-format changelog for the details.

There have been no stable updates over the last week. The,, and updates are in the review process, though, with an expected release on or after July 3. The update contains over 100 patches.

Comments (none posted)

Kernel development news

Quotes of the week

I have a special mix of crack that helps me see Patterns everywhere, even in C code. Some patterns are bright, shiny, and elegant. Others are muddy and confused. struct request_queue has a distinct shadow over it just now.
-- Neil Brown

The whole VM is designed around the notion that most of memory is just clean caches, and it's designed around that simply because if it's not true, the VM freedom is so small that there's not a lot a VM can reasonably do.
-- Linus Torvalds

I'd be quite surprised if they deliberately changed their VFAT code to break Linux with this patch. I'd say it is more likely that once Linux kernels with this change are in widespread use that Microsoft will start to test any changes in their VFAT filesystem to make sure it works with Linux with this patch.
-- Andrew Tridgell

Perhaps we should require that the kernel developers and mainstream distribution maintainers all run Ardour for three weeks and attempt at least two multitrack/multichannel recordings. At least by then they'd maybe have a better notion of what defines a system for serious recording.
-- Dave Phillips

Comments (6 posted)

In brief

By Jonathan Corbet
July 1, 2009
O_NODE. Miklos Szeredi has proposed a new flag (O_NODE) which could be passed to open() calls. This flag, in essence, says that the calling program wants to open the indicated filesystem node, but doesn't want to actually do anything with it. With such opens, the underlying open() file operation will not be called, reads and writes will not be allowed, etc.

One might well wonder what the use for such an operation is. The main motivation would appear to be to allow an application to create a file descriptor which can be passed to other system calls - fstat(), say, or openat(). File descriptors used in this way do not really need access to the underlying file, so it makes sense to provide a way to create file descriptors without that access.

O_PONIES. Rik van Riel has proposed another new open flag (actually called O_REWRITE) which is intended to help applications easily avoid the "zero-length files after a crash" problem. A program could open an existing file with O_REWRITE and get some special semantics. The new file would exist, invisibly, alongside the existing file for as long as it remains open; during that time, any other opens of that file would get the old version. Once the file is closed, the kernel will rename it to the given name in an atomic manner, ensuring that either the old version or the (full) new version will exist should a crash happen in the middle.

This option would make it easy for application developers to rewrite existing files without worrying about robustness. Some might respond that it would be better to just teach those developers to use fsync(), but, as Rik notes, "relying on application developers to get it right seems to not have worked out well." Rik's proposal currently lacks an accompanying patch, so it's not destined for the mainline anytime soon.

VFAT patents. As discussed elsewhere, Andrew Tridgell has posted a new lawyer-approved patch aimed at working around Microsoft's VFAT patents. The discussion on the lists has taken a bit of a different course this time around; there is still some annoyance at making changes like this to deal with the problems of the U.S. patent system, but those voices have been relatively quiet. Not completely quiet, though; Alan Cox said:

Putting the stuff in kernel upsets everyone who isn't under US rule, creates situations where things cannot be discussed and doesn't make one iota of difference to the vendors because they will remove the code from the source tree options and all anyway - because as has already been said it reduces risk.

Beyond that, what developers worry about is interoperability with other VFAT implementations. Alan Cox is asking that, if this patch goes in, the modified filesystem no longer be called "VFAT," since, as he sees it, it's now something else. Ted Ts'o has responded that "VFAT" is a bit of a slippery concept to begin with. It's not clear how this issue will be resolved.

Voyager's voyage. James Bottomley is a proud owner of an archaic Voyager system; Voyager is an x86-based architecture with a number of quaint features - though, contrary to rumor, steam power is not among them. It is not clear whether any Voyager-based systems are still running outside of James's basement. Nonetheless, James has been maintaining the Voyager architecture for years.

More recently, Voyager got kicked out when the code was broken in the process of an x86 subarchitecture-support rewrite. When James tried to get it put back in, x86 Ingo Molnar objected, saying that the costs of the patch were not justified by the benefits of serving such a small user base in the mainline kernel. In the end, Ingo rejected the patch outright, leading to what appeared to be an unsolvable stalemate between the two developers.

Things changed about the time that Linus jumped into the conversation:

Ingo, "absurd irrelevance" is not a reason. If it was, we'd lose about half our filesystems etc. Neither is "thousands of lines of code", or "it hasn't always worked". Again, if it was, then we'd have to get rid of just about all drivers out there.

Ingo eventually backed down on a number of his complaints about the Voyager patches. What remains, though, is a long list of technical problems with the Voyager tree and how it has been managed. James has accepted those complaints as valid, and will work toward resolving them. Before too long, Voyager owners (both of them) should once again have full support for their beloved architecture in the mainline kernel.

Comments (26 posted)

Soft updates, hard problems

July 1, 2009

This article was contributed by Valerie Aurora (formerly Henson)

If a file system discussion goes on long enough, someone will bring up soft updates eventually, usually in the form of, "Duhhh, why are you Linux people so stupid? Just use soft updates, like BSD!" Generally, there will be no direct reply to this comment and the conversation will flow silently around it, like a stream around an inky black boulder. Why is this? Is it pure NIH (Not Invented Here) on the part of Linux developers (and Solaris and OS X and AIX and...) or is there something deeper going on? Why are soft updates so famous and yet so seldom implemented? In this article, I will argue that soft updates are, simply put, too hard to understand, implement, and maintain to be part of the mainstream of file system development - while simultaneously attempting to explain how soft updates work. Oh, the irony!

Soft updates: The view from 50,000 feet

Soft updates is one of a family of techniques for maintaining on-disk file system consistency. The basic problem is that a file system doesn't always get shut down cleanly - think power outage or operating system crash - and if this happens in the middle of an update to the file system (say, deleting a file), the on-disk state of the file system may be inconsistent (corrupt). The original solution to this problem was to run fsck on the entire file system to find and correct inconsistencies; ext2 is an example of a file system that uses this approach. (Note that this use of fsck - to recover from an unclean shutdown - is different from the use of fsck to check and repair a file system that has suffered corruption through some other cause.)

The fsck approach has obvious drawbacks (excessive time, possible lost data), so file system developers have invented new techniques. The most popular and well-known is that of logging or journaling: before we begin writing out the changes to the file system, we write a short description of the changes we are about to make (a journal entry) to a separate area of the disk (the journal). If the system crashes in the middle of writing out the changes, we simply finish up the changes by replaying the journal entry at the next file system mount.

Soft updates, instead, takes a two-step approach to crash recovery. First, we carefully order writes to disk so that, at the time of a crash (or any other time), the only inconsistencies are ones in which a file system structure is marked as allocated when it is actually unused. Second, at the next boot after the crash, fsck is run in the background on a file system snapshot (more on that later) to find and free file system structures that are wrongly marked as allocated. Basically, soft updates orders writes to the disk so that only relatively harmless inconsistencies are possible, and then fixes them in the background by checking and repairing the entire file system. The benchmark results are fairly stunning: in common workloads, performance is often within 5% of that of BSD's memory-only file system. The older version of FFS, which used synchronous writes and foreground fsck to provide similar reliability, often runs 20-30% slower than the in-memory file system.

Step 1: Update dependencies

The first part of implementing soft updates is figuring out how to order the writes to the disk so that after a crash, the only possible errors are inodes and blocks erroneously marked as allocated (when they are actually free). First, the authors lay out some rules to follow when writing changes to disk in order to accomplish this goal. From the paper:

  1. Never point to a structure before it has been initialized (e.g., an inode must be initialized before a directory entry references it).
  2. Never re-use a resource before nullifying all previous pointers to it (e.g., an inode's pointer to a data block must be nullified before that disk block may be re-allocated for a new inode).
  3. Never reset the old pointer to a live resource before the new pointer has been set (e.g., when renaming a file, do not remove the old name for an inode until after the new name has been written).

Pairs of changes in which one change must to be written to disk before the next change can be written, according to the above rules, are called update dependencies. For some more examples of update dependencies, take the case of writing to the first block in a file for the first time. The first update dependency is that the block bitmap, which records which blocks are in-use, must be written to show that the block is in use before the block pointer in the inode is set. If a crash were to occur at this point, the only inconsistency would be one bit in the block bitmap showing a block is allocated when it isn't actually. This is a resource leak, and must be fixed eventually, but the file system can operate correctly with this error as long as it doesn't run out of blocks.

The second update dependency is that the data in the block itself must be written before the block pointer in the inode can be set (along with the increase in the inode size and the associated timestamp updates). If it weren't, a crash at this point would result in garbage appearing in the file - a potential security hole, as well, if that garbage came from a previously written file. Instead, a crash would result in a leaked block (marked as allocated when it isn't) that happens to contain the data from the attempted write. As a result, the write to the bitmap and the write of the data to the block must complete (in any order) before the write that updates the inode's block pointer, size, and timestamps.

These rules about ordering of writes aren't new for soft updates; they were originally created for writes to a "normal" FFS file system. In the original FFS code, ordering of writes is enforced with synchronous writes - that is, the ongoing file system operation (create, unlink, etc.) waits for each ordered write to hit disk before going on to the next step. While the write is in progress, the operating system buffer containing the disk block in question is locked. Any other operation needing to change that buffer has to wait its turn. As a result, many metadata operations progress at disk speed (i.e., murderously slowly).

Step 2: Efficiently satisfying update dependencies

So far, we have determined that synchronous writes on locked buffers are a slow, painful way of enforcing the ordering of writes to the file system. But synchronous writes are overkill for most file system operations; other than fsync(), we generally don't want a guarantee that the result has been written to stable storage before the system call returns, and as we've seen, the file system code itself usually only cares about the order of writes, not when they complete. What we want is a way to record changes to metadata, along with the associated ordering constraints, and then schedule the actual writes at our leisure. No problem, right? We'll just add a couple of pointers to each in-memory buffer containing metadata, linking it to the blocks it has come before and after.

Turns out there is a problem: cyclical dependencies. We have to write to the disk in block-size units, and each block can potentially contain metadata affected by more than one metadata operation. If two different operations affect the same blocks, it can easily result in conflicting requirements: operation A requires that block 1 be written before block 2, and operation B requires that block 2 be written before block 1. Now you can't write out any changes without violating the ordering constraints. What to do?

Most people, at this point, decide to use journaling or copy-on-write to deal with this problem. Both techniques group related changes into transactions - a set of writes that must take effect all at once - and write them out to disk in such a manner that they take effect atomically. But if you are Greg Ganger and Yale Patt, you come up with a scheme to record individual modifications to blocks (such as the update to a single bit in a bitmap block) and their relationships to other individual changes (that change requires this other change to be written out first). Then, when you write out a block, you lock it and iterate through the records of individual changes to this block. For each individual change whose dependencies haven't yet been satisfied, you undo that change to the block, and then write out the resulting block. When the write is done, you re-apply the changes (roll forward), unlock, and continue on your way until the next write. The write you just completed may have satisfied the update dependencies of other blocks, so now you can go through the same process (lock, roll back, write, roll forward, unlock) for those blocks. Eventually, all the dependencies will be satisfied and everything will be written to disk, all without running into any circular dependencies. This, in a nutshell, is what makes soft updates unique.

Recording changes and dependencies

So what does a record of a metadata change, and its corresponding update dependencies, actually look like at the data structure level? First, there are twelve (as of the 1999 paper) distinct structures to record the different types of dependencies:

StructureDependency tracked
bmsafemapblock/inode bitmaps
allocdirectblocks referenced by direct block pointers
indirdepindirect block
allocindirblocks referenced by indirect block pointers
pagedepdirectory entry add/remove
mkdirnew directory creation
dirremdirectory removal
freefragfragment to be freed
freeblksblock to be freed
freefileinode to be freed

Each kind of dependency-tracking structure includes pointers that allow it to be linked into lists attached to the buffers containing the relevant on-disk structures. These lists are what the soft updates code traverses during the roll-back and roll-forward operations on a block being written to disk. Each dependency structure has a set of state flags describing the current status of the dependency. The flags indicate whether the dependency is currently applied to the associated buffer, whether all of the writes it depends on have completed, and whether the update described by the dependency tracking structure itself has been written to disk. When all three of these flags are set (the update is applied to the in-memory buffer, all its dependent writes are completed, and the update is written to disk), the dependency structure can be thrown away.

Page 7 of the 1999 soft updates paper [PDF] begins the descriptions of specific kinds of update dependency structures and their relationships to each other. I've read this paper at least 15 times, and each time I when get to page 7, I'm feeling pretty good and thinking, "Yeah, okay, I must be smarter now than the last time I read this because I'm getting it this time," - and then I turn to page 8 and my head explodes. Here's the first figure on that page:

[Figure 4]

And that's only the figure from the left-hand column. An only slightly less complex spaghetti-gram occupies the right-hand column. This goes on for six pages, describing each specific kind of update dependency and its progression through various lists associated with buffers and file system structures and, most importantly, other update dependency structures. You find yourself struggling through paragraphs like:

Figure 10 shows the structures involved in renaming a file. [Figure 10 looks much like the figure above.] The dependencies follow the same series of steps as those for adding a new file entry, with two variations. First, when a roll-back of an entry is needed because its inode has not yet been written to disk, the entry must be set back to the previous inode number rather than zero. The previous inode number is stored in a dirrem structure. The DIRCHG flag is set in the diradd structure so that the roll-back code knows to use the old inode number stored in the dirrem structure. The second variation is that, after the modified directory entry is written to disk, the dirrem structure is added to the work daemon's tasklist list so that the link count of the old inode will be decremented as described in Section 3.9.

Say that three times fast!

The point is not that the details of soft updates are too complex for mere humans to understand (although I personally wouldn't bet against Greg Ganger being superhuman). The point is that this complexity reflects a lack of generality and abstraction in the design of soft updates as a whole. In soft updates, every file system operation must be individually analyzed for write dependencies, every on-disk structure must have a custom-designed dependency tracking structure, and every update operation must allocate one of these structures and hook itself into the web of other custom-designed dependency tracking structures. If you add a new file system feature, like extended attributes, or change the on-disk format, you have to start from scratch and reason out the relevant dependencies, design a new structure, and write the roll-forward/roll-back routines. It's fiddly, tedious work, and the difficulty of doing it correctly doesn't make it any better a use of programmer staff-hours.

Contrast the highly operation-specific design of soft updates to the transaction-style interface used by most journaling and copy-on-write file systems. When you begin a logical operation (such as a file create), you create a transaction handle. Then, for each on-disk structure you have to modify, you add that buffer to the list of buffers modified by this transaction. When you are done, you close out the transaction and hand it off to the journaling (or COW) subsystem, which figures out how to merge it with other transactions and write them out to disk properly. The user of the transaction interface only has to know how to open, close, and add blocks to a transaction, while the transaction code only has to know which blocks are part of the same transaction. Adding a new write operation requires no special knowledge or analysis beyond remembering to add changed blocks to the transaction.

The lack of generalization and abstraction shows up again when the combination of update dependency ordering and the underlying disk format poke out and cause strange user-visible behavior. The most prominent example shows up when removing a directory; following the rules governing update dependencies means that a directory's ".." entry can't be removed until the directory itself is recorded as unlinked on the disk. Chains of update dependencies sometimes resulted in up to two minutes of delay between the return of rmdir(), and the corresponding decrement of the parent directory's link count. This can break, among other things, a simple recursive "rm -rf". The fix was to fake up a second link count that is reported to userspace, but the real problem was a too-tight coupling between on-disk structures, the system to maintain on-disk consistency, and the user-visible structures. Long chains of update dependencies cause problems elsewhere, during unmount and fsync() in particular.

Fsck and the snapshot

But wait, there's more! The second stage of recovery for soft updates is to run fsck after the next boot, in the background using a snapshot of the file system metadata. File system snapshots in FFS are implemented by creating a sparse file the same size as the file system - the snapshot file. Whenever a block of metadata is altered, the original data is first copied to the corresponding block in the snapshot file. Reads of unaltered blocks in the snapshot redirect to the originals. Online fsck runs on the snapshot of the file system metadata, finding leaked blocks and inodes. Once it completes, fsck uses a special system call to mark these blocks and inodes as free again.

Online fsck implemented in this manner has severe limitations. First, recovery from a crash still requires reading and processing the metadata for the entire file system - in the background, certainly, but that's still a lot of I/O. (Freeblock scheduling piggybacks low-priority I/O, like that of a background fsck, on high-priority foreground I/O so that it interferes as little as possible with "real" work, but that's cold comfort.) Second, it's not actually a full file system check and repair, it's just a scan for leaked blocks and inodes - expected inconsistencies. The whole concept of running fsck on a snapshot file whose blocks are allocated from the same file system assumes that the file system is not corrupted in a way that leaves blocks marked as free when they are actually allocated.


Conceptually, soft updates can be explained concisely - order writes according to some simple rules, track updates to metadata blocks, roll-back updates with unsatisfied dependencies before writing the block to disk, then roll-forward the updates again. But when it come to implementation, only programmers with deep, encyclopedic knowledge of the file system's on-disk format can derive the correct ordering rules and construct the associated data structures and web of dependencies. The close coupling of on-disk data structures and their updates and the user-visible data structures and their updates results in weird, counter-intuitive behavior that must be covered up with additional code.

Overall, soft updates is a sophisticated, insightful, clever idea - and an evolutionary dead end. Journaling and copy-on-write systems are easier to implement, require less special-purpose code, and demand far less of the programmer writing to the interface.

Comments (40 posted)

Perfcounters added to the mainline

By Jake Edge
July 1, 2009

We last looked at the perfcounters patches back in December, shortly after they appeared. Since that time, a great deal of work has been done, culminating in perfcounters being included into the mainline during the recently completed 2.6.31 merge window. Along the way, a tool to use perfcounters, called perf, was added to the mainline as well.

Adding perf to the kernel tools/ directory is one of the more surprising aspects of the perfcounters merge. Kernel hackers have long been leery of adding user-space tools into the kernel source tree, but Linus Torvalds was unconvinced by multiple complaints about that approach. He pointed to oprofile to explain:

It took literally months for the user mode tools to catch up and get the patches to support new functionality into CVS (or is it SVN?), and after that it took even longer for them to become part of a release and be picked up by distributions. In fact, I'm not sure it is part of a release even now - I had to make a bug report to Fedora to get atom and Nehalem support in my tools: I think they took the unofficial patch.

Others were not so sure that the oprofile being developed separately from the kernel was the root cause of those failures. Christoph Hellwig had other ideas: "I don't think oprofile has been a [disaster] because of any kind of split, but because the design has been a failure from day 1." But, Torvalds wants to try including the tool to see where it leads: "Let's give a _new_ approach a chance, and see if we can avoid the mistakes of yesteryear this time."

The perf tool itself is a fairly simple command-line tool, which can be built from the tools/perf directory. It also includes some documentation, in the form of man pages that are also available via perf help (as well as in HTML and other formats). At its simplest, it gathers and reports some statistics for a particular command:

    $ perf stat ./hackbench 10
    Time: 4.174

     Performance counter stats for './hackbench 10':

	8134.135358  task-clock-msecs     #      1.859 CPUs
	      23524  context-switches     #      0.003 M/sec
	       1095  CPU-migrations       #      0.000 M/sec
	      16964  page-faults          #      0.002 M/sec
	10734363561  cycles               #   1319.669 M/sec
	12281522014  instructions         #      1.144 IPC
	  121964514  cache-references     #     14.994 M/sec
	   10280836  cache-misses         #      1.264 M/sec

	4.376588249  seconds time elapsed.
This summarizes the performance events that occurred while running the hackbench micro-benchmark program. There are a combination of hardware events (cycles, instructions, cache-references, and cache-misses) as well as software events (task-clock-msecs, context-switches, CPU-migrations, and page-faults) that are derived from the kernel code and not the processor-specific performance monitoring unit (PMU). Currently, support for hardware events is available for Intel, AMD, and PowerPC PMUs, but other architectures still have support for the software events.

There is also a top-like mode for observing which kernel functions are being executed most frequently in a continuously updating display:

    $ perf top -c 1000 -p 3216

       PerfTop:     360 irqs/sec  kernel:65.0% [1000 cycles],  (target_pid: 3216)

		 samples    pcnt         RIP          kernel function
      ______     _______   _____   ________________   _______________

		 1214.00 -  5.3% - 00000000c045cb4d : lock_acquire
		 1148.00 -  5.0% - 00000000c045d1d3 : lock_release
		  911.00 -  4.0% - 00000000c045d377 : lock_acquired
		  509.00 -  2.2% - 00000000c05a0cbc : debug_locks_off
		  490.00 -  2.2% - 00000000c05a2f08 : _raw_spin_trylock
		  489.00 -  2.1% - 00000000c041d1d8 : read_hpet
		  488.00 -  2.1% - 00000000c04419b8 : run_timer_softirq
		  483.00 -  2.1% - 00000000c04d5f72 : do_sys_poll
		  477.00 -  2.1% - 00000000c05a34a0 : debug_smp_processor_id
		  462.00 -  2.0% - 00000000c043df85 : __do_softirq
		  404.00 -  1.8% - 00000000c074d93f : sub_preempt_count
		  353.00 -  1.5% - 00000000c074d9d2 : add_preempt_count
		  338.00 -  1.5% - 00000000c0408a76 : native_sched_clock
		  318.00 -  1.4% - 00000000c074b4c3 : _spin_lock_irqsave
		  309.00 -  1.4% - 00000000c044ea10 : enqueue_hrtimer
This is a static version of the output from looking at a largely quiescent firefox process (pid 3216), sampling every 1000 cycles.

There is quite a bit more that perf can do. There is a record sub-function that gathers the performance counter data into a file which can be used by other commands:

    $ perf record ./hackbench 10
    Time: 4.348
    [ perf record: Captured and wrote 2.528 MB (~110448 samples) ]

    $ perf report --sort comm,dso,symbol | head -15

    # (110146 samples)
    # Overhead           Command  Shared Object              Symbol
    # ........  ................  .........................  ......
	10.70%         hackbench  [kernel]                   [k] check_bytes_and_report
	 9.07%         hackbench  [kernel]                   [k] slab_pad_check
	 5.67%         hackbench  [kernel]                   [k] on_freelist
	 5.28%         hackbench  [kernel]                   [k] lock_acquire
	 5.03%         hackbench  [kernel]                   [k] lock_release
	 3.19%         hackbench  [kernel]                   [k] init_object
	 3.02%         hackbench  [kernel]                   [k] lock_acquired
	 2.47%         hackbench  [kernel]                   [k] _raw_spin_trylock
This output shows the top eight kernel functions executed while running hackbench. The same data file can also be used by perf annotate (when given a symbol name and the appropriate vmlinux file) to show the disassembled code for a function, along with the number of samples recorded on each instruction. There is clearly a wealth of information that can be derived from the tool.

The original posting of the perfcounters patches came as somewhat of a surprise to Stéphane Eranian, who had long been working on another performance monitoring solution, "perfmon". While he is still a bit skeptical of perfcounters, which were originally proposed by Ingo Molnar and Thomas Gleixner, he has been reviewing the patches, and providing lengthy comments. Molnar, also responded at length, breaking his reply into multiple chunks which can be found in the thread.

Perfmon was targeted at exposing as much of the underlying PMU data as possible to user space, but Molnar explicitly rejects that goal:

So for every "will you support advanced PMU feature X, Y and Z" question you ask, the first-level answer is: 'please show the developer usecase and integrate it into our tools so we can see how it all works and how useful it is'.

"A tool might want to do this" is not a good enough answer. We now have a working OSS tool-space with 'perf' where such arguments for more PMU features can be made in very specific terms: patches, numbers and comparisons. Actual hands-on utility, happy developers and faster apps is what matters in the end - not just the list of PMU features we expose.

His focus, presumably shared with his co-maintainers Peter Zijlstra and Paul Mackerras, is to generalize performance measurement features so that they are not dependent on any particular CPU and that they fit well with developer work flow: "I do claim we had few if any sane performance analysis tools before under Linux, and i think we are still in the stone ages and still have a lot of work to do in this area." From Molnar's perspective, that ease of use for users and developers is one of the main areas where perfmon fell short.

Molnar is not shy about pointing out that perfcounters still needs a lot of work, but the framework is there, so features can be added to that. As yet, there is no documentation in the kernel Documentation/ directory, but one presumes that will be handled sometime soon. Overall, perfcounters and the perf tool look to be a highly useful addition to the kernel, one that should start providing benefits—in the form of better performance—in the near term.

Comments (18 posted)

The fanotify API

By Jonathan Corbet
July 1, 2009
One of the features merged for 2.6.31 is the "fsnotify" file event notification framework. Fsnotify serves as a new, common underpinning for the inotify and dnotify APIs, simplifying the code considerably. But this simplification, as welcome as it is, was never the real purpose behind fsnotify. Instead, fsnotify exists to serve as the support structure for fanotify, the "fscking all notification system," which has now been posted for further review.

Fanotify was once known as TALPA; its main purpose is to enable the implementation of malware scanners on Linux systems. When TALPA was first proposed, it ran into criticism on a number of fronts, not the least of which being a general disdain for malware scanning as a security technique. The sad fact of the matter, though, is that a number of customers require this functionality, so a market for such products on Linux exists. Thus far, scanning products for Linux have relied on a number of distasteful techniques, including hooking into the system call table or the loading of binary-only security modules. Fanotify, it is hoped, will help to wean these products off of such hacks and get them out of the kernel altogether.

The user-space API used by fanotify is, to your editor's eye, a little strange. An fanotify application starts by opening a socket with the new PF_FANOTIFY protocol family. This socket must then be bound to an "address" described this way:

    struct fanotify_addr {
	sa_family_t family;
	__u32 priority;
	__u32 group_num;
	__u32 mask;
	__u32 timeout;
	__u32 unused[16];

The family field should be AF_FANOTIFY. The priority field is used to determine which socket gets a specific event if more than one fanotify socket exists; lower priority values win. The group_num is used by the fsnotify layer to identify a group of event listeners. The timeout field currently appears to be unused. Finally, mask describes the events that the application is interested in hearing about:

  • FAN_ACCESS: every file access.
  • FAN_MODIFY: file modifications.
  • FAN_CLOSE: when files are closed.
  • FAN_OPEN: open() calls.
  • FAN_ACCESS_PERM: like FAN_ACCESS, except that the process trying to access the file is put on hold while the fanotify client decides whether to allow the operation.
  • FAN_OPEN_PERM: like FAN_OPEN, but with the permission check.
  • FAN_EVENT_ON_CHILD: the caller is interested in events on full directory hierarchies.
  • FAN_GLOBAL_LISTENER: notify for events on all files in the system.

Once the socket has been bound, the application can learn about filesystem activity using the well-known event-reading system call getsockopt(). A call to getsockopt() with SOL_FANOTIFY as the level and FANOTIFY_GET_EVENT as the option will return one or more structures like this:

    struct fanotify_event_metadata {
	__u32 event_len;
	__s32 fd;
	__u32 mask;
	__u32 f_flags;
	pid_t pid;
	pid_t tgid;
	__u64 cookie;

Here, fd is an open, read-only file descriptor for the file in question, mask describes the event (using the flags described above), f_flags is a copy of the flags provided by the process trying to access the file, and pid and tgid identify that process (in a namespace-unaware way, currently). If the event is one requiring permission from the application, cookie will contain a value which can be used to grant or deny that permission.

Note that the provided file descriptor should eventually be closed; otherwise these file descriptors are likely to accumulate rather quickly.

When access decisions are being made, the application must notify the kernel with a call to setsockopt() using the FANOTIFY_ACCESS_RESPONSE option and a structure like:

    struct fanotify_so_access {
	__u64 cookie;
	__u32 response;

The cookie value from the event should be provided, and response should be one of FAN_ALLOW or FAN_DENY. If the application does not respond within five seconds, the kernel will allow the action to proceed. Five seconds should be sufficient for file scanning, but it could be a problem with some other possible applications of fanotify, such as hierarchical storage management systems. Fanotify developer Eric Paris notes that a future option allowing the response to be delayed indefinitely will probably be added at some point.

It is possible to adjust the set of files subject to notifications with the FANOTIFY_SET_MARK, FANOTIFY_REMOVE_MARK, and FANOTIFY_CLEAR_MARKS operations. If the FAN_GLOBAL_LISTENER option was provided at bind time, then all files are "marked" at the outset; FANOTIFY_REMOVE_MARK can be used to prune those which are not interesting. Otherwise at least one FANOTIFY_SET_MARK call must be made before events will be returned.

Some details have been left out, but the above discussion covers the core parts of the fanotify API. Comments on this posting have been relatively scarce; opposition to this feature seems to have faded away over the last year or so. What's left is getting the API right; your editor suspects that the use of getsockopt() as an event retrieval interface may raise a few eyebrows sooner or later. But, once that's ironed out, chances are good that Linux will be well on the way toward having a general file access notification and permission interface.

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