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 415823B6351; Mon, 23 Mar 2026 14:01:20 +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=1774274480; cv=none; b=VN0K7hGDAS9law1bna/W1qRhFMh3CR0MzDq9TQotWb/dKd5yS31t/0m6DDosNP9Xm2qReDNK+Ikvj1D9SU4sy8aaSOZV7ZXevXvBB/ZNN2YkJh7qDxVVVqRgSDCuhLY0QxgTAGUx4fkZWfCgQ6NUbFH0Pp3NeMISEv8PYwGfbJY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774274480; c=relaxed/simple; bh=120DJpSQwN94ULG1GMyl/YyLuQ4BQodrvTRi+gQqfZw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=r+P9yxaCPJwQNIjOtkg8ezBgMWPlPaQxDuIsTjipDFAxi7NhoN7Q2jkVbAPquFdQn95z/BxdDNG79La7oCp8B/3ICUF7o+tQfd4i0Z4ZmHzxzO7ot1W3nNX6IVlNTYbdGGwIcb0NLwggpZf3cKaSDH9G5fyTCarYGV7H58XaYLU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=o0TjX1wt; 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="o0TjX1wt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 89182C4CEF7; Mon, 23 Mar 2026 14:01:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774274480; bh=120DJpSQwN94ULG1GMyl/YyLuQ4BQodrvTRi+gQqfZw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=o0TjX1wts7VexcRxHQ68DZvIgYEOgfPSm+qjK2OqmFgKCBpe52p+WuDtOl5NUMc/4 lRRjRKozIL0NC5eGAv+IyDmHVId0MUHb4+yevF0jHQxPsMEvOnwP4MVHYztcGj2xKw Y6IppdCl0yI7ugSuGpsZQlU5NpyYJY+Zgz33l0Sw= 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 6.18 011/212] mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared THP Date: Mon, 23 Mar 2026 14:43:52 +0100 Message-ID: <20260323134504.124697151@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134503.770111826@linuxfoundation.org> References: <20260323134503.770111826@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 6.18-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 @@ -2346,11 +2346,17 @@ static bool try_to_migrate_one(struct fo /* PMD-mapped THP migration entry */ if (!pvmw.pte) { 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 subpage = folio_page(folio,