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imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=FmNAkM8P; spf=pass (imf06.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 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=1770237829; a=rsa-sha256; cv=none; b=ZUKUQUECds4/t7iEPm6p/xS6hVZ7orw/4YfC9KDThG/oZKTMmZSyj/2kmCdXbJe1cZWwyu IyH5BB+MJzbNe2Z78Vp9SISrUClDXSWXUvXwq/3UT18QdzWtH7ZARmTrqr5e/8C0eUFsO0 WI2I3E9jP22k0hYzV6334HVsnwjszdI= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 742A14398D; Wed, 4 Feb 2026 20:43:47 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id CB711C4CEF7; Wed, 4 Feb 2026 20:43:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770237827; bh=KWSG6PWanp5dDS8uJVMx5rjP8FzfGf3/ZzBMjhgV2us=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=FmNAkM8PJqKCYA2j+QcaaxVSh/GQI62iNvi9qqTy29je5IVbNYHuS9Y76g5ccjw2e DdP6cjPjrL+P4RrglwS2Bw0/kcXaLWuYzP7KmF8ShPaqM8KGTsKVSF++5AiEEXUisR 57nIvYwX20tWGjTkHSDtfZAfWN9DGtjhYZfM41Ci9zp/MQGxaMqb80k9OaVa7UOqDF 3AwMiWZArvxgOqndgNBYMpFC6HaqpsOXNnOkHHrxa3sfRJ5woBNjGOuY1HwBWsIIBe b3Acm4Qm0cHnbbcdjlwU6ZC1Q0omVMKbWy8+O+IgkVMltkCsITtW+z8ilsFoAl0c0V Qu4WWUuZiUR/w== Message-ID: Date: Wed, 4 Feb 2026 21:43:42 +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: Zi Yan Cc: 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, gavinguo@igalia.com, baolin.wang@linux.alibaba.com, 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: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: D67EE180006 X-Stat-Signature: ujzai7wyfum3tg43e4kqyhzrtpcmm3hs X-Rspam-User: X-HE-Tag: 1770237828-940465 X-HE-Meta: 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 Re18RbxD 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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: On 2/4/26 21:02, Zi Yan wrote: > On 4 Feb 2026, at 14:36, David Hildenbrand (arm) wrote: > >> 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? > > The explanation is not for the shared case mentioned above. It is for unshared > folio. If an unshared folio’s PAE cannot be cleared, try_to_migrate_one() return > true, indicating a success. Oh. You mean that should be something like "This patch fixes this by restart page_vma_mapped_walk() after split_huge_pmd_locked(). We cannot simply return "true" to fix the problem, as that would affect another case: split_huge_pmd_locked()->folio_try_share_anon_rmap_pmd() can failed and leave the folio mapped through PTEs; we would return "true" from try_to_migrate_one() in that case as well. While that is mostly harmless, we could end up walking the rmap, wasting some cycles.". > Yeah, since it is an unshared folio, the return > value of try_to_migrate_one() does not matter. This fix makes try_to_migrate_one() > return false. Right, it's not really problematic. We could end up walking the rmap and burn some cycles. > >> >> >> (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. > > The commit message might be improved with some additional context. The comment > above pairs with the comment in __split_huge_pmd_locked() > “In case we cannot clear PageAnonExclusive(), split the PMD > only and let try_to_migrate_one() fail later”. What is problem with it? With your explanation it's much clearer, thanks. I'd remove some details from the comments about PAE like: "split_huge_pmd_locked() might leave the folio mapped through PTEs. Retry the walk so we can detect this scenario and properly abort the walk." With some clarifications along those lines Acked-by: David Hildenbrand (arm) -- Cheers, David