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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 14 Apr 2023 16:49:19 -0700
Message-ID: <CAJuCfpH278ozvhz=z4W0d6TxY6HhrORbsBPFM5d4x1M=EM23WQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm: do not increment pgfault stats when page fault
 handler retries
To:     Peter Xu <peterx@redhat.com>
Cc:     akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org,
        mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz,
        ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com,
        vbabka@suse.cz, minchan@google.com, dave@stgolabs.net,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kernel-team@android.com
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On Fri, Apr 14, 2023 at 3:35=E2=80=AFPM Peter Xu <peterx@redhat.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=3D=3DVM_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
>