linux-kernel - Re: [PATCH 06/25] mm/arm64: Use mm_fault

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Date:   Tue, 16 Jun 2020 11:59:14 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 06/25] mm/arm64: Use mm_fault_accounting()

Hi, Will,

On Tue, Jun 16, 2020 at 08:43:08AM +0100, Will Deacon wrote:
> Please can you explain why it's ok to move the PERF_COUNT_SW_PAGE_FAULTS
> update like this? Seems like a user-visible change to me, so some
> justification would really help.

Indeed this could be a functional change for PERF_COUNT_SW_PAGE_FAULTS on some
archs, e.g., for arm64, PERF_COUNT_SW_PAGE_FAULTS previously will also contain
accounting of severe errors where we go into the "no_context" path.  However if
you see the other archs, it's not always true, for example, the xtensa arch
only accounts the correctly handled faults (arch/xtensa/mm/fault.c).

After I thought about this, I don't think it's extremely useful (or please
correct me if I missed something important) to use PERF_COUNT_SW_PAGE_FAULTS
for fatal error accountings among all those correct ones.  After all they are
really extremely rare cases, and even if we got a sigbus for a process, we'll
normally got something dumped in dmesg so if we really want to capture the
error cases there should always be a better way (because by following things
like dmesg we can not only know how many error faults triggered, but also on
the details of the errors).

IOW, my understanding of users of PERF_COUNT_SW_PAGE_FAULTS is that they want
to trap normal/correct page faults, not really care about rare errors.

Then when I went back to think PERF_COUNT_SW_PAGE_FAULTS, it's really about:

  A=PERF_COUNT_SW_PAGE_FAULTS 
  B=PERF_COUNT_SW_PAGE_FAULTS_MAJ
  C=PERF_COUNT_SW_PAGE_FAULTS_MIN

And:

  A=B+C

If that's the case (which is simple and clear), it's really helpful too that we
unify this definition across all the architectures, then it'll also be easier
for us to provide some helper like mm_fault_accounting() so that the accounting
can be managed in the general code rather than in arch-specific ways.

Thanks,

-- 
Peter Xu