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imf11.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=R34k9rUv; spf=pass (imf11.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770233783; a=rsa-sha256; cv=none; b=FJTz3z4uCPfi/DhoP8GLkeEG7FXggemw7GnOMiG5KgEIMg4Aoh8pdGQiw4mJjn/e1Mho7x jwUtOiUEkBxk9n5YKjEgXyKpGoF0nJgoFqvzxkMf0jK6RfWAFq6w5NdevSALd582nx2HlT 0MJNKo8CpApbXBYPfMnJMWcEfxMJmuo= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 9A9FF60018; Wed, 4 Feb 2026 19:36:22 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D92C9C4CEF7; Wed, 4 Feb 2026 19:36:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770233782; bh=QU3jrWsGIuZ9o2iKsCUnTMZ1BLOu3kxDvnNgU0Q6PbY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=R34k9rUvrSuNHWvpokBkg5c1iGffIx40UJz0bBuQO2uqidy8D5Hh3vxwD5PzHVfsj nW28jOkm5TfKDV8kT/dm/8hvYVaYLD1NzPsq0XGiDQxGKrd7PSLYlBrWPSSFKOyY+C pZ5bJ0VmHcqmagN/VlHHdmWW35iw8X7sYJ1lWiPet4wje/wnx6Pt53yjKx/KhxdMTZ ZR0y+BDzRVipQ8sPc/y/ess5IJ2zydaSQzhYI1Jh9xlKATMpYq8fneH2wj75r8ZEwv VfH0BZRyG/Xim/MxAfcZ7LuwlFJiRxfO6edj5Yp5bGsbRLMDoVHNZ0TVSCXHSGwtUa q3uQPohPCSEFA== Message-ID: Date: Wed, 4 Feb 2026 20:36:16 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [Patch v2] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared thp To: Wei Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, jannh@google.com, ziy@nvidia.com, gavinguo@igalia.com, baolin.wang@linux.alibaba.com Cc: linux-mm@kvack.org, Lance Yang , stable@vger.kernel.org References: <20260204004219.6524-1-richard.weiyang@gmail.com> From: "David Hildenbrand (arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 47E0240004 X-Stat-Signature: 6mzf1bj9qq3xms6z8zbc44goqkpc1fqj X-Rspam-User: X-HE-Tag: 1770233783-109962 X-HE-Meta: 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 R2ozIlet 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Sorry for the late reply. I saw that I was CCed in v1 but I am only now catching up with mails ... slowly but steadily. > Without the above commit, we can successfully split to order 0. > With the above commit, the folio is still a large folio. > > The reason is the above commit return false after split pmd > unconditionally in the first process and break try_to_migrate(). > > The tricky thing in above reproduce method is current debugfs interface > leverage function split_huge_pages_pid(), which will iterate the whole > pmd range and do folio split on each base page address. This means it > will try 512 times, and each time split one pmd from pmd mapped to pte > mapped thp. If there are less than 512 shared mapped process, > the folio is still split successfully at last. But in real world, we > usually try it for once. Ah, that explains magic number 513. > > This patch fixes this by restart page_vma_mapped_walk() after > split_huge_pmd_locked(). Because split_huge_pmd_locked() may fall back to > (freeze = false) if folio_try_share_anon_rmap_pmd() fails and the PMD is > just split instead of split to migration entry. Right, but folio_try_share_anon_rmap_pmd() should never fail on the folios that have already been shared? (above you write that it is shared with 512 children) The only case where folio_try_share_anon_rmap_pmd() could fail would be if the folio would not be shared, and there would only be a single PMD then, so there is nothing you can do -> abort. Returning "false" from try_to_migrate_one() is the real issue, as it makes rmap_walk_anon() to just stop -> abort the walk. So I suspect v1 was actually sufficient, or what am I missing where the restart would actually be required? (maybe we should get rid of the usage of booleans here at some point, an enum like abort/continue would have been much clearer) > Restart > page_vma_mapped_walk() and let try_to_migrate_one() try on each PTE > again and fail try_to_migrate() early if it fails. > > Signed-off-by: Wei Yang > Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") > Cc: Gavin Guo > Cc: "David Hildenbrand (Red Hat)" > Cc: Zi Yan > Cc: Baolin Wang > Cc: Lance Yang > Cc: > > --- > v2: > * restart page_vma_mapped_walk() after split_huge_pmd_locked() > --- > mm/rmap.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 618df3385c8b..5b853ec8901d 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -2446,11 +2446,16 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, > __maybe_unused pmd_t pmdval; > > if (flags & TTU_SPLIT_HUGE_PMD) { > + /* > + * After split_huge_pmd_locked(), restart the > + * walk to detect PageAnonExclusive handling > + * failure in __split_huge_pmd_locked(). > + */ > split_huge_pmd_locked(vma, pvmw.address, > pvmw.pmd, true); > - ret = false; > - page_vma_mapped_walk_done(&pvmw); > - break; > + flags &= ~TTU_SPLIT_HUGE_PMD; > + page_vma_mapped_walk_restart(&pvmw); > + continue; > } The change looks more consistent to what we have in try_to_unmap(). But the explanation above is not quite right I think. And consequently the comment above as well. PAE being set implies "single PMD" -> unshared. -- Cheers, David