From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793306600; b=Fb8iwdoL5p6t0/FlX+ZBwWJfBOZ5UodRwh3UCuo4V8uhkkC0DC2kj5y1DWQs6bRU6Uw4 pNKHnQZwXmr6Nap7FM9mMt+5l/t8p0Kada9MYFTdDSeemqknYCbQjnP/F8RYvgzCWZuJ8 M3FDiuUTOyIWBYEk56tgSv7B7yLFFp8ptPmtV2iXGkqUxMoZZyESpD7ZvcsCPsvVlRbAk xou2lKeecwyfrDXI5W/uJqTfrp5jT4ipC9nXrc6e+rsXfnCYk3b5aCUXf9ClZY3Vw4SpI 9oFb7I8DD2Zjs6SpdAKTnt8qsQwRklccdVqi4UDly531/frIB1h8dRD4FHq4ixtYUbQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793306600; h=Received:DKIM-Signature:From:Date:Subject:MIME-Version:Content-Type: Content-Transfer-Encoding:Message-Id:References:In-Reply-To:To:Cc: X-Mailer:X-Developer-Signature:X-Developer-Key; bh=yQnqLaEhsJZDZQiQXLxV9X+lF54eRYn7Ww6geWf1QZw=; b=SFMDzBNaSbK7IZuPqEZtDoapLXdwuyGsIFLMSH94FUhr64zIwCnWZusoYSvfrblyCzKJ CutEOI/EeEuspRpi5oaeETeZRVaYPA9QdssuJmKfV023GUau5zq/kGJhr/HT+kVyJHAiX 3yBOmJ9XNq9lN5HhBtzgwOQXrDfjJEbaGh/keKcORepQdZysC4xaLdwvksLedFWUai9Ma RIggGmuEL+BUEVdNsiVfwDCc/+P+5SX6xypPNf/nZIY73UnhzZwiUfv7i80ukCWPLLUFD +wpemhovvRH7OAivtUbDtCQq5G90NuSXNtDiE+KDw/1x+pBHpOXzrkd/UNgeAIW4/qA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=lxeF63YQ; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F2A6A606A31F; Thu, 29 Oct 2026 20:43:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=lxeF63YQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 243251F01816; Thu, 29 Oct 2026 20:43:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793306599; bh=yQnqLaEhsJZDZQiQXLxV9X+lF54eRYn7Ww6geWf1QZw=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=lxeF63YQFJJ/+XQ1nKhqIj0w0bilMMDyUEqz6TAJmSfD4k1gY2szSiICglDwDHga6 VIiOBv00CLMPJJ+o6rva1soNmT+ykIjzUf577nQbaOGc9An7nSRhke1U4NTx+4scdq b8sKytj+/S3XoeL/oRoAEBIa5MHsZaSbq5G0FBLZecxAdpTkhTMGik1bT6p0rr2KEP 04HE5nXsIqQ0tw+lwb7wjhObeN2E7nsrhgYUwr9xbYZwtD7RoCjxHJCezR+jdFTgbw eu7TE8Q3HQh25zZ+90tK5nrxcqL8piZpV5vwZvHzg4lcE8Fo09OIMp7yCBjVLt1wkq xzz859cxecnXA== From: "David Hildenbrand (Arm)" Date: Thu, 29 Oct 2026 21:33:35 +0100 Subject: [PATCH v2 40/47] mm/ksm: fix use of a freed anon_vma at swap-in X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Message-Id: <20261029-fixes_core-v2-40-a64ce7f83abb@kernel.org> References: <20261029-fixes_core-v2-0-a64ce7f83abb@kernel.org> In-Reply-To: <20261029-fixes_core-v2-0-a64ce7f83abb@kernel.org> To: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, stable@vger.kernel.org, Chris Mason , "David Hildenbrand (Arm)" X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5952; i=david@kernel.org; h=from:subject:message-id; bh=kkxSpuAzd3ayVbIamzbiuyVrNHFCuPg3p8/sm8anyic=; b=owGbwMvMwCF2/OVpCdfQ3ZWMp9WSGLIer+1kabPS4ml+lj7Zefdyh2rD4jurrQobvDpzfjOce zTXZM6UjlIWBjEOBlkxRZag4PsGL1+uq61242OGmcPKBDKEgYtTACZyfSrDPzMRQzGRA8u8ZjF/ WmbRKPdcT/fU73K5m+dnqBxiMnPo2svwV8zj5YI7hxvPbIu5XXrqyp0FW9y4VzGltzLsX+5l9ly UkQUA X-Developer-Key: i=david@kernel.org; a=openpgp; fpr=1BD9CAAD735C4C3A460DFCCA4DDE10F700FF835A A folio read from a swap slot previously shared by KSM can remain in the swap cache after the process that first mapped it exits. Its anon_vma can then be freed, so ksm_might_need_to_copy() must not dereference it to compare roots. Doing so can result in some false-positives, resulting in the reverse mapping missing the new folio mapping, preventing migration and reclaim. SLAB_TYPESAFE_BY_RCU allows individual anon_vma objects to be reused immediately after freeing. A stale pointer can then refer to another anon_vma whose root matches, even though the target VMA is not linked to that anon_vma. Move the anonymous rmap compatibility checks into folio_anon_rmap_compatible(). Keep the page-index check and compare the folio's anon_vma pointer against the VMA's own anon_vma and root first. If neither matches, bite the bullet and walk the VMA's anon_vma_chain. The VMA keeps these anon_vmas alive, and finding a match guarantees that reverse mapping can find the new mapping. Lorenzo Stoakes says in a private discussion: "This isn't quite equivalent to the existing check. Every anon_vma in an anon_vma_chain shares the same root, but if you split a VMA then fork both of them like this: VMA A / \ VMA B VMA C \ / root <- anon_vma 1 ----^ ^---- / parent \ anon_vma 2 anon_vma 3 | | VMA D VMA E VMA's D and E both get their own new anon_vma's such that: VMA D: anon_vma 2 -> anon_vma 1 -> root VMA E: anon_vma 3 -> anon_vma 1 -> root And if you found a folio referencing anon_vma 2, the existing check: anon_vma->root == vma->anon-vma->root Would be true for VMA's B, C, D & E. However after your change it'd be true for VMA D only, as it's the only one with anon_vma 2 in its anon_vma_chain. Whether or not that could actually happen in practice... 🙂 I don't know. [...] after your change it'd be true for VMA D only, as it's the only one with anon_vma 2 in its anon_vma_chain. [...] I would [...] say that you are making the test stricter but more correct. " If this affects performance when KSM was never even involved, we could just track whether KSM ever merged anything and just skip these checks. There isn't really a lot we can do otherwise: for mapped folios we might be able to use folio_get_anon_vma(), but for unmapped folios there really is no easy way to see if the anon_vma is still valid. Document the (confusing) lifetime requirements of folio_anon_vma(). Fixes: ba6f0ff3981e ("ksm: fix ksm swapin time optimization") Cc: stable@vger.kernel.org Reported-by: Chris Mason Reviewed-by: Lorenzo Stoakes (ARM) Signed-off-by: David Hildenbrand (Arm) --- mm/ksm.c | 36 +++++++++++++++++++++++++++++++----- mm/util.c | 13 +++++++++++++ 2 files changed, 44 insertions(+), 5 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index 9c51925825cb..e050b1bdf810 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -3137,11 +3137,40 @@ void __ksm_exit(struct mm_struct *mm) trace_ksm_exit(mm); } +static bool folio_anon_rmap_compatible(const struct folio *folio, + struct vm_area_struct *vma, unsigned long addr) +{ + const struct anon_vma *anon_vma = folio_anon_vma(folio); + struct anon_vma_chain *avc; + + VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio); + VM_WARN_ON_ONCE_FOLIO(folio_test_large(folio), folio); + VM_WARN_ON_ONCE_FOLIO(folio_test_ksm(folio), folio); + vma_assert_stabilised(vma); + + if (!anon_vma) + return true; + if (folio->index != linear_anon_page_index(vma, addr)) + return false; + + /* + * Careful: anon_vma might have been freed, so we cannot safely + * dereference it. Add a fast path before we walk the anon_vma chain. + */ + if (anon_vma == vma->anon_vma || anon_vma == vma->anon_vma->root) + return true; + + list_for_each_entry(avc, &vma->anon_vma_chain, same_vma) { + if (avc->anon_vma == anon_vma) + return true; + } + return false; +} + struct folio *ksm_might_need_to_copy(struct folio *folio, struct vm_area_struct *vma, unsigned long addr) { struct page *page = folio_page(folio, 0); - struct anon_vma *anon_vma = folio_anon_vma(folio); struct folio *new_folio; if (folio_test_large(folio)) @@ -3151,10 +3180,7 @@ struct folio *ksm_might_need_to_copy(struct folio *folio, if (folio_stable_node(folio) && !(ksm_run & KSM_RUN_UNMERGE)) return folio; /* no need to copy it */ - } else if (!anon_vma) { - return folio; /* no need to copy it */ - } else if (folio->index == linear_anon_page_index(vma, addr) && - anon_vma->root == vma->anon_vma->root) { + } else if (folio_anon_rmap_compatible(folio, vma, addr)) { return folio; /* still no need to copy it */ } if (PageHWPoison(page)) diff --git a/mm/util.c b/mm/util.c index 8acfe2148ba5..80f6c714844f 100644 --- a/mm/util.c +++ b/mm/util.c @@ -697,6 +697,19 @@ void *vcalloc_noprof(size_t n, size_t size) } EXPORT_SYMBOL(vcalloc_noprof); +/** + * folio_anon_vma - Get a folio's raw anon_vma pointer. + * @folio: The folio. + * + * The returned pointer may be stale for an unmapped folio. The caller + * must ensure the anon_vma stays alive before dereferencing it. A folio + * reference or lock alone is insufficient; use folio_get_anon_vma() when + * the anon_vma's lifetime is not otherwise protected. Note that + * folio_get_anon_vma() is only useful for folios that are still mapped. + * + * Return: The raw anon_vma pointer for an anonymous, non-KSM folio, or + * %NULL otherwise. + */ struct anon_vma *folio_anon_vma(const struct folio *folio) { unsigned long mapping = (unsigned long)folio->mapping; -- 2.43.0