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Date: Sun, 28 Jun 2026 19:11:58 -0700
From: Andi Kleen <andi@firstfloor.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, David Hildenbrand <david@kernel.org>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Naoya Horiguchi <nao.horiguchi@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Oscar Salvador <osalvador@suse.de>,
	Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>,
	Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@kernel.org>,
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	David Rientjes <rientjes@google.com>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Harry Yoo <harry@kernel.org>, Hao Li <hao.li@linux.dev>,
	Kiryl Shutsemau <kas@kernel.org>, Byungchul Park <byungchul@sk.com>,
	linux-mm@kvack.org, linux-cxl@vger.kernel.org,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with
 non-atomic page flag ops
Message-ID: <akHUboUaYi-OJHFu@firstfloor.org>
References: <cover.1782676497.git.mst@redhat.com>
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In-Reply-To: <cover.1782676497.git.mst@redhat.com>

On Sun, Jun 28, 2026 at 05:45:22PM -0400, Michael S. Tsirkin wrote:
> This series fixes the race by:
> 
> 1. Having memory_failure() call synchronize_rcu() + retry after
>    setting HWPoison, so that any in-flight non-atomic RMW that
>    read the old flags value completes before we proceed.
>
> 2. Wrapping all non-atomic page flag operations in
>    rcu_read_lock/rcu_read_unlock (CONFIG_MEMORY_FAILURE only),
>    so that synchronize_rcu() actually drains them.

It wouldn't surprise me if your underlying performance assumptions 
-- an non contended atomic is cheaper than a rcu_read_lock/unlock --
are not true in various CPU/kernel configuration combinations.

Modern CPUs have fast atomics when uncontended in normal circumstances.

But it probably doesn't matter much either way because the difference
shouldn't be very much.  It seems very complicated for something that
could be much simpler.

But I guess it's fine.

-Andi

> 
> Performance impact (page alloc+free microbenchmark, 200K iterations,
> 20 runs, KVM guest, error bars are 3-sigma):
> 
>   !PREEMPT_RCU (x86):
>                          insns/iter      cycles/iter
>     base:                12237 +/- 1     17954 +/- 136
>     patched:             +22 +/- 1       -124 +/- 122
>                          (+0.18%)        (within noise)
> 
>   PREEMPT_RCU:
>                          insns/iter      cycles/iter
>     base:                12512 +/- 3     18541 +/- 214
>     patched:             +95 +/- 3       -12 +/- 161
>                          (+0.76%)        (within noise)
> 
> When !CONFIG_MEMORY_FAILURE, all wrappers compile away completely.