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Sanitizing kernel memorySanitizing kernel memoryPosted Jun 2, 2009 15:29 UTC (Tue) by etienne_lorrain@yahoo.fr (guest, #38022)In reply to: Sanitizing kernel memory by anton Parent article: Sanitizing kernel memory
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.
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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 (subscriber, #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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