linux-kernel - Re: [PATCH 1/1] mm: do not increment pgfault stats when page fault handler retries

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Date:   Fri, 14 Apr 2023 16:49:19 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/1] mm: do not increment pgfault stats when page fault
 handler retries

On Fri, Apr 14, 2023 at 3:35 PM Peter Xu <peterx@...hat.com> wrote:
>
> Hi, Suren,
>
> On Fri, Apr 14, 2023 at 03:14:23PM -0700, Suren Baghdasaryan wrote:
> > > It also already ignores invalid faults:
> > >
> > >         if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY))
> > >                 return;
> >
> > Can there be a case of (!VM_FAULT_ERROR && VM_FAULT_RETRY) - basically
> > we need to retry but no errors happened? If so then this condition
> > would double-count pagefaults in such cases.
>
> If ret==VM_FAULT_RETRY it should return here already, so I assume
> mm_account_fault() itself is fine regarding fault retries?
>
> Note that I think "ret & (VM_FAULT_ERROR | VM_FAULT_RETRY)" above means
> "either ERROR or RETRY we'll skip the accounting".
>
> IMHO we should have 3 cases here:
>
>   - ERROR && !RETRY
>     error triggered of any kind
>
>   - RETRY && !ERROR
>     we need to try one more time
>
>   - !RETRY && !ERROR
>     we finished the fault

After looking some more into mm_account_fault(), I think it would be
fine to count the faults which produced errors. IIUC these counters
represent the total number of faults, not the number of valid and
successful faults. If so then I think simply using VM_FAULT_RETRY
should be ok without considering all possible combinations. WDYT?

>
> I don't think ERROR & RETRY can even be set at the same time so I assume
> there's no option 4) - a RETRY should imply no ERROR already, even though
> it's still incomplete so need another attempt.
>
> Thanks,
>
> --
> Peter Xu
>