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

[Michel Lespinasse]

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.
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