Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

Peter Zijlstra
2010-11-17T15:29:03Z

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.