From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 6BD603806B8; Tue, 14 Jul 2026 14:58:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784041122; cv=none; b=urxSI0jO9C3yDX+TT6uRvGazteA3VD0jSUQ3aCfWoR3y+CDsI/RCeiIsir7PbYSGk8TcuDlMihGu8jnJwRakeDFrshrIk+rrYwgD0uBkwqImf6SyeNBGWDZWpGpaS4HMlZwK7l4lrqOmJUxjWkiN4pda2oagYYTjKJDyr0lKppc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784041122; c=relaxed/simple; bh=crNfwxjYmfbBRjp1JNdkVG2Bccrjzdqcg8iEAjXEvUc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nQRuSPoi5uHBR6xabaMBAt19/whHlJOby8AKg2yP7i4A9dLSt37aJ4MD48ATffLyPl7BMrHmckStTFetBDIzqe1+4i2lRWepRjpms5zljICtkV+joPqmgZD+l+GAcgyCh4I3EZlE75aMCXeRudKllpfznNqXzvFwWCmE+4K159c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GRM3rkZt; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GRM3rkZt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 321A31F00A3A; Tue, 14 Jul 2026 14:58:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784041121; bh=/rDw+3jasEMQfWDR5/QI5ODPO0V9yA5DbFG2otOFc44=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=GRM3rkZtto20bP3/Luu98NsJmLDkagWlrqsOixM8cvwNyxbbeaLw5UIoaiRzufdXT KWMeO9vlv+CN38PSZJYzRmPrecqS/9elUGuXIIRUZ6X/IFUUZ+TSpAtpSkvzot5fW/ edQqYyE+wqRS+d2jUg1noS/tJDlsioUV07fohRUwI9zDrQGhiS8NVoWzvegJ3PE5iX Gav5cG+5LPW46u96Cr6is8LYT+jyPZcVJg1yT9jAd0Lz1SYlTRVrcohaBGymOYSWFl W1Da1iowMFfia0NZNLdEbAupRIiX54l4jr0Voa4QeJeM4P1c2NUL9R57RbXoBrtPjf Fkt/XC1qhYLow== Message-ID: <18fe5529-2ad5-4330-a362-708a152bacee@kernel.org> Date: Tue, 14 Jul 2026 16:58:36 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/5] mm/memory-failure: keep the folio, not the poisoned subpage, locked across split To: Kiryl Shutsemau Cc: Andrew Morton , Lorenzo Stoakes , Miaohe Lin , Naoya Horiguchi , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Hao Zhang , Hao Zhang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260714122344.351895-1-kirill@shutemov.name> <20260714122344.351895-2-kirill@shutemov.name> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/14/26 16:53, Kiryl Shutsemau wrote: > On Tue, Jul 14, 2026 at 03:01:46PM +0200, David Hildenbrand (Arm) wrote: >> On 7/14/26 14:23, Kiryl Shutsemau wrote: >>> From: "Kiryl Shutsemau (Meta)" >>> >>> try_to_split_thp_page() locked the poisoned page and passed it to >>> split_huge_page_to_order(), which returns that very page locked to the >>> caller. For a tail page that means __folio_split() runs with @lock_at >>> pointing into the middle of the folio. >>> >>> __folio_split() dereferences the mapping after the split completes >>> (shmem_uncharge(), i_mmap_unlock_read()). The only thing keeping the >>> inode alive across that is the locked @lock_at folio: while it stays in >>> the page cache, eviction cannot complete. >>> >>> But a tail @lock_at can lie beyond EOF -- e.g. part of a shmem THP that >>> reaches past i_size while the file is being truncated. The split then >>> drops it from the page cache yet still returns it locked, so the pin is >>> gone and a racing final iput() can evict and RCU-free the inode while >>> __folio_split() is still running: >>> >>> BUG: KASAN: slab-use-after-free in __up_read+0x634/0x790 >>> i_mmap_unlock_read include/linux/fs.h:537 [inline] >>> __folio_split+0x732/0x1640 mm/huge_memory.c:4100 >>> try_to_split_thp_page+0xab/0x390 mm/memory-failure.c:1675 >>> memory_failure+0x1394/0x26e0 mm/memory-failure.c:2470 >>> >>> Freed by task 4601: >>> shmem_free_in_core_inode+0x54/0xb0 mm/shmem.c:5177 >>> evict+0x57f/0xac0 fs/inode.c:870 >>> >>> Split the folio as a folio, via split_folio_to_order(), so the head is >>> the anchor left locked. The head is piece 0, which the beyond-EOF drop >>> loop never removes (it starts at folio_next(folio)), so the split always >>> leaves it in the page cache and the inode stays pinned for the whole of >>> __folio_split(). memory_failure() and soft offline re-lock the poisoned >>> subpage's folio themselves after the split, so they do not depend on it >>> being returned locked. >>> >>> Reported-by: Hao Zhang >>> Closes: https://lore.kernel.org/linux-mm/20260710071344.GA106129@zh-pc >>> Fixes: baa355fd3314 ("thp: file pages support for split_huge_page()") >>> Cc: >>> Signed-off-by: Kiryl Shutsemau (Meta) >>> --- >>> mm/memory-failure.c | 13 ++++++++++--- >>> 1 file changed, 10 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c >>> index 51508a55c405..68d42cbed458 100644 >>> --- a/mm/memory-failure.c >>> +++ b/mm/memory-failure.c >>> @@ -1657,11 +1657,18 @@ static int identify_page_state(unsigned long pfn, struct page *p, >>> static int try_to_split_thp_page(struct page *page, unsigned int new_order, >>> bool release) >>> { >>> + struct folio *folio = page_folio(page); >>> int ret; >>> >>> - lock_page(page); >>> - ret = split_huge_page_to_order(page, new_order); >>> - unlock_page(page); >>> + /* >>> + * Lock and split at the head, not the poisoned subpage: __folio_split() >>> + * keeps the anchor folio locked and needs it to stay in the page cache >>> + * to pin the inode. A tail beyond EOF would be dropped yet returned >>> + * locked, losing that pin. The caller re-locks @page afterwards. >>> + */ >>> + folio_lock(folio); >>> + ret = split_folio_to_order(folio, new_order); >>> + folio_unlock(folio); >> >> With a non-uniform split it would actually make a difference: we'd want to split >> such that we the other folio pages minimal. >> >> split_folio_to_order() always seems to end up in >> __split_huge_page_to_list_to_order() where we do a SPLIT_TYPE_UNIFORM. >> >> I recall discussing with Zi and Willy that in the future we'd want to convert >> more places to do a non-uniform split. >> >> So I'm afraid that would just re-introduce the problem then. > > Right. Non-uniform split can be useful. > > But my patch is completely broken because code expects the pin to be on the > @page, not on the head. put_page() few lines down can explode already. Yeah. > > So the fix does not belong in memory_failure(). It belongs in > __folio_split(), and it is really just 2/5: refuse the split with -EBUSY > when @lock_at is at or beyond the sampled EOF. > > The safety then sits in __folio_split() regardless of caller or split type, > which should also cover your non-uniform worry. > > The behavioural change is that memory_failure() reports a beyond-EOF > poisoned tail as unsplit (MF_FAILED) instead of recovered, and kills the > mappers instead of splitting the page off. What we give up is salvaging > the folio's healthy pages and the clean unmap -- both low value for a page > that is beyond EOF and getting truncated away. Containment is unaffected: > PageHWPoison is set before the split and free_pages_prepare() keeps a > poisoned page out of the buddy allocator, so the bad page never comes back > regardless. > > The alternative, if we would rather keep the recovered outcome, is to leave > @lock_at as the poisoned page and move i_mmap_unlock_read() (and > shmem_uncharge()) ahead of the after-split unlock loop, so every mapping > dereference happens while @folio -- the head, within EOF -- still pins the > inode. That is close to Hao's original patch. It works, but it rests on "no > mapping dereference after the unlock loop", which is its own fragility. At least it can be well documented. > > I lean towards the -EBUSY guard. The latter approach seems cleaner to me, but let's hear others as well. -- Cheers, David