From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B0C32199FAB; Mon, 23 Mar 2026 13:49:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774273793; cv=none; b=oip1GFKhRQjiJWa5VaullkbLWeQyQr2q6oqwfUS/Xtx1S3iE5Ie6Ceeois+FILftq9JzxO5xMWm7IqwFlmwXFR1O9yus++KWYn4S3DlZH4VTs34xVOGrxF+auFxw/dVX2UsDfpDbhQ48hnPjvfmS0+LTZ6U3erVHhmk0RRWNDec= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774273793; c=relaxed/simple; bh=eG8v+v94y11ftNPYiTYPIy1Gq7J1KgH8yKnUAOcEWaw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=e0vRMlbttcjnuiF7OTsYnJKV6EoXBaMzqTuGdH+DM9av5qdpMT+DIfqnPhkN2Tyv3LFcoyvmn41X2/zssVJmnYxeZpJSIIPg3G7LSXBKwHRkMAn1/fSQLoZPt/R2+Zt66LPOwQFzWYjWZfab8/AijXr0Dr9FRM3NYDaHSb3tca8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=RIkK/God; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="RIkK/God" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BC729C2BCB1; Mon, 23 Mar 2026 13:49:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774273793; bh=eG8v+v94y11ftNPYiTYPIy1Gq7J1KgH8yKnUAOcEWaw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=RIkK/God/7pEvfHsfpGxaJ2JHUcbQW5N7TMDI9IY8kM1TLCI9uKsoxyB8wopl9Hjv SlnXjxZb2f+NfU/b3huC/9a2q3bKe8k9eLoukze72rFKXfXm10v8eyQDLPpkiba6OZ 56cvZnmCUTCTpWCIEYQ7RoTfKYBtY/nz8DKKkMTU= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Wei Yang , Baolin Wang , Zi Yan , Lance Yang , Gavin Guo , "David Hildenbrand (arm)" , "Lorenzo Stoakes (Oracle)" , Andrew Morton Subject: [PATCH 6.19 011/220] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared THP Date: Mon, 23 Mar 2026 14:43:08 +0100 Message-ID: <20260323134504.937875443@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134504.575022936@linuxfoundation.org> References: <20260323134504.575022936@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.19-stable review patch. If anyone has any objections, please let me know. ------------------ From: Wei Yang commit 939080834fef3ce42fdbcfef33fd29c9ffe5bbed upstream. Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") return false unconditionally after split_huge_pmd_locked(). This may fail try_to_migrate() early when TTU_SPLIT_HUGE_PMD is specified. The reason is the above commit adjusted try_to_migrate_one() to, when a PMD-mapped THP entry is found, and TTU_SPLIT_HUGE_PMD is specified (for example, via unmap_folio()), return false unconditionally. This breaks the rmap walk and fail try_to_migrate() early, if this PMD-mapped THP is mapped in multiple processes. The user sensible impact of this bug could be: * On memory pressure, shrink_folio_list() may split partially mapped folio with split_folio_to_list(). Then free unmapped pages without IO. If failed, it may not be reclaimed. * On memory failure, memory_failure() would call try_to_split_thp_page() to split folio contains the bad page. If succeed, the PG_has_hwpoisoned bit is only set in the after-split folio contains @split_at. By doing so, we limit bad memory. If failed to split, the whole folios is not usable. One way to reproduce: Create an anonymous THP range and fork 512 children, so we have a THP shared mapped in 513 processes. Then trigger folio split with /sys/kernel/debug/split_huge_pages debugfs to split the THP folio to order 0. Without the above commit, we can successfully split to order 0. With the above commit, the folio is still a large folio. And currently there are two core users of TTU_SPLIT_HUGE_PMD: * try_to_unmap_one() * try_to_migrate_one() try_to_unmap_one() would restart the rmap walk, so only try_to_migrate_one() is affected. We can't simply revert commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()"), since it removed some duplicated check covered by page_vma_mapped_walk(). This patch fixes this by restart page_vma_mapped_walk() after split_huge_pmd_locked(). Since we cannot simply return "true" to fix the problem, as that would affect another case: When invoking folio_try_share_anon_rmap_pmd() from split_huge_pmd_locked(), the latter can fail and leave a large folio mapped through PTEs, in which case we ought to return true from try_to_migrate_one(). This might result in unnecessary walking of the rmap but is relatively harmless. Link: https://lkml.kernel.org/r/20260305015006.27343-1-richard.weiyang@gmail.com Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") Signed-off-by: Wei Yang Reviewed-by: Baolin Wang Reviewed-by: Zi Yan Tested-by: Lance Yang Reviewed-by: Lance Yang Reviewed-by: Gavin Guo Acked-by: David Hildenbrand (arm) Reviewed-by: Lorenzo Stoakes (Oracle) Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- mm/rmap.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2338,11 +2338,17 @@ static bool try_to_migrate_one(struct fo __maybe_unused pmd_t pmdval; if (flags & TTU_SPLIT_HUGE_PMD) { + /* + * 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. + */ 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; } #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION pmdval = pmdp_get(pvmw.pmd);