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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 24 Jun 2020 12:50:55 -0700
Message-ID: <CALvZod6BEw-1=XgwN6RDohnEp1uwmDQwPPvgbtC2H4q-X=Nsig@mail.gmail.com>
Subject: Re: [PATCH] mm: filemap: clear idle flag for writes
To:     Yang Shi <yang.shi@linux.alibaba.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Rik van Riel <riel@surriel.com>, gavin.dg@linux.alibaba.com,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 24, 2020 at 12:18 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>
>
> On 6/24/20 11:53 AM, Andrew Morton wrote:
> > On Thu, 25 Jun 2020 01:43:32 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
> >
> >> Since commit bbddabe2e436aa7869b3ac5248df5c14ddde0cbf ("mm: filemap:
> >> only do access activations on reads"), mark_page_accessed() is called
> >> for reads only.  But the idle flag is cleared by mark_page_accessed() so
> >> the idle flag won't get cleared if the page is write accessed only.
> >>
> >> Basically idle page tracking is used to estimate workingset size of
> >> workload, noticeable size of workingset might be missed if the idle flag
> >> is not maintained correctly.
> >>
> >> It seems good enough to just clear idle flag for write operations.
> >>
> >> ...
> >>
> >> --- a/mm/filemap.c
> >> +++ b/mm/filemap.c
> >> @@ -41,6 +41,7 @@
> >>   #include <linux/delayacct.h>
> >>   #include <linux/psi.h>
> >>   #include <linux/ramfs.h>
> >> +#include <linux/page_idle.h>
> >>   #include "internal.h"
> >>
> >>   #define CREATE_TRACE_POINTS
> >> @@ -1630,6 +1631,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
> >>
> >>      if (fgp_flags & FGP_ACCESSED)
> >>              mark_page_accessed(page);
> >> +    else if (fgp_flags & FGP_WRITE) {
> >> +            /* Clear idle flag for buffer write */
> >> +            if (page_is_idle(page))
> >> +                    clear_page_idle(page);
> >> +    }
> >>
> >>   no_page:
> >>      if (!page && (fgp_flags & FGP_CREAT)) {
> > The kerneldoc comment for pagecache_get_page() could do with some
> > updating - it fails to mention FGP_WRITE, FGP_NOFS and FGP_NOWAIT.
>
> Yes, will propose a separate patch later on.
>
> >
> > This change seems correct but also will have runtime effects.  What are
> > they?
>
> Other than a couple of extra cycles when idle page tracking is enabled,
> I didn't think of other effects. It should be negligible. The idle flag
> doesn't play a role in page reclaim algorithm, so it won't have impact
> on that.
>
>

The only user visible impact will be on idle page tracking users. They
will get more accurate data.