linux-kernel - Re: [PATCH 1/1] arch/mm/fault: fix major fault accounting when retrying under per-VMA lock

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Date: Sun, 21 Jan 2024 23:38:38 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: patchwork-bot+linux-riscv@...nel.org, linux-riscv@...ts.infradead.org, 
	akpm@...ux-foundation.org, willy@...radead.org, will@...nel.org, 
	catalin.marinas@....com, palmer@...belt.com, mpe@...erman.id.au, 
	christophe.leroy@...roup.eu, agordeev@...ux.ibm.com, 
	gerald.schaefer@...ux.ibm.com, dave.hansen@...ux.intel.com, luto@...nel.org, 
	peterz@...radead.org, x86@...nel.org, linux-arm-kernel@...ts.infradead.org, 
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] arch/mm/fault: fix major fault accounting when
 retrying under per-VMA lock

On Sat, Jan 20, 2024 at 1:15 PM Russell King (Oracle)
<linux@...linux.org.uk> wrote:
>
> On Sat, Jan 20, 2024 at 09:09:47PM +0000, patchwork-bot+linux-riscv@...nel.org wrote:
> > Hello:
> >
> > This patch was applied to riscv/linux.git (fixes)
> > by Andrew Morton <akpm@...ux-foundation.org>:
> >
> > On Tue, 26 Dec 2023 13:46:10 -0800 you wrote:
> > > A test [1] in Android test suite started failing after [2] was merged.
> > > It turns out that after handling a major fault under per-VMA lock, the
> > > process major fault counter does not register that fault as major.
> > > Before [2] read faults would be done under mmap_lock, in which case
> > > FAULT_FLAG_TRIED flag is set before retrying. That in turn causes
> > > mm_account_fault() to account the fault as major once retry completes.
> > > With per-VMA locks we often retry because a fault can't be handled
> > > without locking the whole mm using mmap_lock. Therefore such retries
> > > do not set FAULT_FLAG_TRIED flag. This logic does not work after [2]
> > > because we can now handle read major faults under per-VMA lock and
> > > upon retry the fact there was a major fault gets lost. Fix this by
> > > setting FAULT_FLAG_TRIED after retrying under per-VMA lock if
> > > VM_FAULT_MAJOR was returned. Ideally we would use an additional
> > > VM_FAULT bit to indicate the reason for the retry (could not handle
> > > under per-VMA lock vs other reason) but this simpler solution seems
> > > to work, so keeping it simple.
> > >
> > > [...]
> >
> > Here is the summary with links:
> >   - [1/1] arch/mm/fault: fix major fault accounting when retrying under per-VMA lock
> >     https://git.kernel.org/riscv/c/46e714c729c8
> >
> > You are awesome, thank you!
>
> Now that 32-bit ARM has support for the per-VMA lock, does that also
> need to be patched?

Yes, I think so. I missed the ARM32 change that added support for
per-VMA locks. Will post a similar patch for it tomorrow.
Thanks,
Suren.

>
> --
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