Re: [PATCH] mm: don't flush TLB when propagate PTE access bit to struct page.

[Hugh Dickins <hughd@google.com>]

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On Wed, 27 Oct 2010, Nick Piggin wrote:
> On Wed, Oct 27, 2010 at 12:22 PM, Nick Piggin <npiggin@gmail.com> wrote:
> > On Wed, Oct 27, 2010 at 12:05 PM, Rik van Riel <riel@redhat.com> wrote:
> >> On 10/27/2010 01:21 PM, Ying Han wrote:
> >>>
> >>> kswapd's use case of hardware PTE accessed bit is to approximate page LRU.
> >>>  The
> >>> ActiveLRU demotion to InactiveLRU are not base on accessed bit, while it
> >>> is only
> >>> used to promote when a page is on inactive LRU list.  All of the state
> >>> transitions
> >>> are triggered by memory pressure and thus has weak relationship with
> >>> respect to
> >>> time.  In addition, hardware already transparently flush tlb whenever CPU
> >>> context
> >>> switch processes and given limited hardware TLB resource, the time period
> >>> in
> >>> which a page is accessed but not yet propagated to struct page is very
> >>> small
> >>> in practice. With the nature of approximation, kernel really don't need to
> >>> flush TLB
> >>> for changing PTE's access bit.  This commit removes the flush operation
> >>> from it.

It should at least add a comment there in page_referenced_one(), that
a TLB flush ought to be done, but is now judged not worth the effort.

(I'd expect architectures to differ on whether it's worth the effort.)

> >>>
> >>> Signed-off-by: Ying Han<yinghan@google.com>
> >>> Singed-off-by: Ken Chen<kenchen@google.com>

Hey, Ken, switch off those curling tongs :)

> However, it's a scary change -- higher chance of reclaiming a TLB covered page.

Yes, I was often tempted to make such a change in the past;
but ran away when it appeared to be in danger of losing the pte
referenced bit of precisely the most intensively referenced pages.

Ying's point (about what the pte referenced bit is being used for in our
current implementation) is interesting, and might have tipped the balance;
but that's not clear to me - and the flush is only done when mm is on CPU.

> 
> I had a vague memory of this problem biting someone when this flush wasn't
> actually done properly... maybe powerpc.
> 
> But anyway, same solution could be possible, by flushing every N pages scanned.

Yes, batching seems safer.

Hugh