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

Nick Piggin
2010-11-17T12:58:22Z

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.

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).