On Wed, Nov 17, 2010 at 7:28 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>