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Sanitizing kernel memorySanitizing kernel memoryPosted May 28, 2009 18:25 UTC (Thu) by anton (guest, #25547)In reply to: Sanitizing kernel memory by lemmings Parent article: Sanitizing kernel memory Clearing on freeing will also add a performance cost in the zeroed case: Even if you avoid the cache and CPU cost of clearing in some way, there are still the memory bandwidth costs of writing the zeros out, and later loading them again; plus the CPU latency cost of having to load the zeros into the cache on use. In contrast, if you zero the page on-demand, the zeros are written to the cache, and in many cases are then available to the user program from the cache, requiring less CPU time and less memory bandwidth.
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Sanitizing kernel memory Posted May 28, 2009 18:38 UTC (Thu) by gmaxwell (subscriber, #30048) [Link]
My thought process was the that the writing at free is cheap (especially as you can avoid pulling in anything in from memory or evicting anything from the cache) and that any userspace app that reads memory handed to it by the kernel before writing it is broken. Don't these people have valgrind?
:)
Sanitizing kernel memory Posted May 28, 2009 19:09 UTC (Thu) by anton (guest, #25547) [Link] any userspace app that reads memory handed to it by the kernel before writing it is broken.If the application writes a byte to a line that's not in cache, current CPUs load (the rest of) the cache line from main memory (write allocate). In a few special cases this read can be avoided, but for ordinary writes it happens.
Sanitizing kernel memory Posted May 30, 2009 10:10 UTC (Sat) by willezurmacht (guest, #58372) [Link]
"any userspace app that reads memory handed to it by the kernel before writing it is broken. Don't these people have valgrind?"
Or maybe it's an exploit for an information/memory leak bug in the kernel that retrieves your root password and tty buffers contents from memory.
Sanitizing kernel memory Posted May 28, 2009 22:06 UTC (Thu) by nix (subscriber, #2304) [Link]
Would a DMA engine help here? A lot of modern chipsets have them
(sometimes they even work).
Sanitizing kernel memory Posted May 29, 2009 8:50 UTC (Fri) by anton (guest, #25547) [Link] A DMA engine would "avoid the cache and CPU cost of clearing" (that's what I was thinking about when I wrote that), but the other costs would still be there.
Sanitizing kernel memory Posted Jun 2, 2009 15:29 UTC (Tue) by etienne_lorrain@yahoo.fr (guest, #38022) [Link]
On the other side, if a function allocates more than it will use (allocating the maximum size is quite usual, and *alloc() may round up the size), then the zeroing at allocation will pollute all those very performance enhancing 32+ bytes cache 1 lines...
Moreover, DMA zeroing at free() time should set those cache lines in a state where they will be first reused (instead of more important lines). Sorry, not statistics available.
Sanitizing kernel memory Posted Jun 2, 2009 16:24 UTC (Tue) by anton (guest, #25547) [Link] Yes, if the zeroed cache lines are not accessed or not accessed before being replaced in the cache, then the performance of on-demand zeroing for that cache line will be just as bad as eager zeroing in the cache, and a little worse than for eager methods that don't go through the cache. But I doubt that that's the case for the majority of cache lines. In particular, I don't think that there are replaced at all cache levels before being accessed. But yes, measurements would be a good idea.Concerning eager zeroing of cache lines, that is certain to replace a page full of cache lines just as on-demand zeroing does, except that it is far less likely that the cache lines will be accessed before being replaced by other cache lines, so it is a bad idea. Tagging the line as least-recently-used helps only a little, if it is possible at all.
Sanitizing kernel memory Posted Jun 4, 2009 18:30 UTC (Thu) by oak (guest, #2786) [Link]
> On the other side, if a function allocates more than it will use
I think all the user-space allocated pages point to the same zeroed/shared
Sanitizing kernel memory Posted Jun 9, 2009 12:28 UTC (Tue) by etienne_lorrain@yahoo.fr (guest, #38022) [Link]
> user-space allocated pages
We were talking about kernel-space allocated memory... Having a stock of cleared pages for user-space would probably also be an improvement (when user-space writes its first byte on a page), as long as clearing those pages does not wipe the memory cache (i.e. DMA to memory instead of processor writing).
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