Various short topics

Discussion on linux-kernel this week touched on a number of topics that, while worth a mention, don't necessarily justify a full article of their own. Here's a few of them:

Deprecating the .gz format. Peter Anvin would like to get rid of the .gz files on kernel.org. The bzip2 format has been around for quite some time and is far more space efficient; it would seem that eliminating the older format would be relatively uncontroversial. Such is not the case, however; users protested that the bzip2 format is slower, is not supported on Windows, and so on. The end result is that the gzip files will remain for some time yet.

kbugs.org is now showing over 1400 potential bugs found with the rapidly-evolving smatch system. A number of these are real, and fixes are beginning to find their way into the mainline kernel.

The Stanford Checker team has also been posting errors; the latest set points out places where kernel code is directly dereferencing user-supplied pointers. That kind of mistake can lead to all kinds of problems, of course, including security issues. The discussion led to a suggestion that the kernel use a different type for user-space pointers, so that this kind error could be caught directly by the compiler. The idea makes some sense; kernel code currently does not formally distinguish between user-space, kernel-space, and physical address pointers. Clarifying the difference between them could catch a lot of mistakes. This sort of change seems unlikely at this point in 2.5, however.

The object-based reverse mapping VM patch was covered here back in February. The object-based rmap code does not work with anonymous memory (memory which is not mapped to a file somewhere), however, meaning that this memory must still be managed with PTE chains. Hugh Dickins has posted a new set of patches which extend the object-based approach to anonymous memory as well. The patch was included in the -mm tree for a while, and seems to work without trouble. The only problem is: it doesn't actually help performance very much. Most anonymous memory only shows up in one page table, so its PTE chain overhead is essentially zero. So this patch has been dropped, though useful pieces of it may eventually find their way into the tree.

The IDE todo list has been posted by Alan Cox. This list is important in that it affects most Linux users; it also documents some of the remaining tasks to be done on the way to a 2.6 release. There's a few drivers needing thorough audits (and some that don't work at all yet), more hotplug work, documentation, and a number of other tasks yet to be done.

SMP overhead and rwlocks. Andrew Morton has noted that a simple write test takes twice as long on an SMP system as on a uniprocessor system. The culprit, of course, is the extra locking overhead. Reader-writer locks (rwlocks) have been singled out as particular problem; it turns out that they are slower than regular spinlocks, and they tend to mask problems where locks are simply being held for too long. There is a chance that rwlocks will be removed before 2.6 comes out.

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