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t=1786468324; bh=pZIR+kTb1vaEP0SOLFxQlJy7QQ3zoJxZgV/8LQbwkY8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=C5fsfUytqsw6kTo91U7cmiAYC9oANteHMadRxEPieLDJ0gdltUPlCQ1AZ/vLa6UQj Qh1BHXHkEqSs3eqSqsvhlAKJbGayJ8POlvdEkBVfkShvr56pdl6W6PnmPZ9psH0xt+ 5aIYPYDajwYV7zeokBSfCoabO+L9+nDnpVa0hxLZOcDhjaa8LHgA8LNWlpDBbFEBKA kO5PZjC8W8Ud9xQv6Op5YXQ/D1ApixDbuLl2QhjgnOff7enPjil3aOTN3bkc73ptyc 4PatbiL0ST/QfckmNFnsPQwtQR1P4W6nGTt32raaoQOhD2K/tJ2qTd+rEJvFFieEbM QrCBAqGXwlu4w== Message-ID: Date: Tue, 11 Aug 2026 19:11:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] selftests/mm: add stale walk->action race test To: Hyunwoo Kim , akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260811161949.3879321-1-imv4bel@gmail.com> <20260811161949.3879321-3-imv4bel@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7F0FE40016 X-Stat-Signature: fjdc319dirt44xzxk47anks1hdf6j55e X-HE-Tag: 1786468325-111939 X-HE-Meta: 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 U3dbxYPZ eb+JIUHWMqycITqFeIlHpjKT0lEkvUmxR8w/hEvkqR2mYciiOvGT0lx+1aXQAiYaMkCVWmkmBlaGkSNT1wUo+GTcFSJybPE//Fef1MEFF1j0f/Fu8DjiEJemRM+h9/FRUZnGKOiXLX+itPZPp8/cFvr9+Bnqhe9m7R5wcgi7dGmMiDkE1I+TwUzJC6PBq2vU26UcTpa0XUR6rEwirBt5IWcsoi8WM2Hrwr6YecvMQ/xMFTMEOlkbJH1xlAqnsvYzE9lxLvHypAkl3ifg9iBHjWQs8+A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/11/26 18:18, Hyunwoo Kim wrote: > The added pagewalk_race_test maps two PMDs and faults in 2MB of the first > one. A second thread then faults in the second PMD and drops it again with > MADV_DONTNEED in a loop, while the main thread reads Rss for the mapping > from /proc/self/smaps. > > Clearing the second PMD while smaps_pte_range() runs leaves walk->action > erroneously set to ACTION_AGAIN, which causes the PUD walk to be retried, > so the first PMD is counted twice and Rss comes out twice as large as what > was faulted in. > > mincore() is the caller named in the fix, but the second walk writes past > the length mincore() copies back, so it cannot be seen from userspace > there. smaps reports what the callbacks counted, so the duplicate shows up > in Rss. > > A failure can only come from the kernel counting the same page twice, so > missing the race is harmless. On an unfixed kernel the test fails after a > few hundred reads at most and takes about half a second. > > Assisted-by: Claude:claude-opus-5 > Signed-off-by: Hyunwoo Kim > --- I'm sorry, but I don't consider such a specialized reproducer for a problem we hit once a good fit for a selftest. Or am I missing something, that this here is more generic? -- Cheers, David