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 18:34:57 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.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

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

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