Messages in this thread | | | Date | Wed, 17 Nov 2010 14:05:30 -0800 | Subject | Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback | From | Michel Lespinasse <> |
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On Wed, Nov 17, 2010 at 7:28 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Wed, 2010-11-17 at 23:57 +1100, Nick Piggin wrote: >> On Wed, Nov 17, 2010 at 04:23:58AM -0800, Michel Lespinasse wrote: >> > When faulting in pages for mlock(), we want to break COW for anonymous >> > or file pages within VM_WRITABLE, non-VM_SHARED vmas. However, there is >> > no need to write-fault into VM_SHARED vmas since shared file pages can >> > be mlocked first and dirtied later, when/if they actually get written to. >> > Skipping the write fault is desirable, as we don't want to unnecessarily >> > cause these pages to be dirtied and queued for writeback. >> >> It's not just to break COW, but to do block allocation and such >> (filesystem's page_mkwrite op). That needs to at least be explained >> in the changelog. > > Agreed, the 0/3 description actually does mention this. > >> Filesystem doesn't have a good way to fully pin required things >> according to mlock, but page_mkwrite provides some reasonable things >> (like block allocation / reservation). > > Right, but marking all pages dirty isn't really sane. I can imagine > making the reservation but not marking things dirty solution, although > it might be lots harder to implement, esp since some filesystems don't > actually have a page_mkwrite() implementation.
Really, my understanding is that not pre-allocating filesystem blocks is just fine. This is, after all, what happens with ext3 and it's never been reported as a bug (that I know of).
If filesystem people's feedback is that they really want mlock() to continue pre-allocating blocks, maybe we can just do it using fallocate() rather than page_mkwrite() callbacks ?
-- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.
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