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

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Message-ID: <Zaw31DVa9q3JZASo@shell.armlinux.org.uk>
Date: Sat, 20 Jan 2024 21:15:01 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: patchwork-bot+linux-riscv@...nel.org
Cc: Suren Baghdasaryan <surenb@...gle.com>, 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 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?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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