From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f45.google.com (mail-pj1-f45.google.com [209.85.216.45]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E59E63AA4F8 for ; Sat, 5 Sep 2026 09:58:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.45 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788602333; cv=none; b=J97sFiNhIx/R+DfzTxphUnWeYgj7ky5JaFZqzeNekJU8lxwPjWV39Qi0yIR4mpresc+8sGnTed/F3RjYU+0tqfLutEhhR2t29w3WSh+VDqtKLHpU/JTp8PxXYmLzIlBrJKqbsuGZ5vYcuk223sKaEyMGEoisOEFGTLt+Xx9dSHo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788602333; c=relaxed/simple; bh=Yl5fK/RbIDJdvoRdwQIojnzqdqNNfQohYVikRsBK1X0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oAVx+JM4DgX4TH0myo0rgM2HCnWkOi2Vj4jCLrN8HAOL7thD4heHH/ySURo9xZ9gVquoZSSmC7HSg2DrNv47SBi7W49urwba9ItlTUlzKBWYc9qhuYtELmFQN/fw8LRXMzqwCrT63SVy6nm7Krrdr0j8rGRCiHC/3CjqyWWBKbI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=a3NybSZh; arc=none smtp.client-ip=209.85.216.45 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="a3NybSZh" Received: by mail-pj1-f45.google.com with SMTP id 98e67ed59e1d1-38a0c7e841fso2312813a91.2 for ; Sat, 05 Sep 2026 02:58:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1788602331; x=1789207131; darn=vger.kernel.org; h=content-transfer-encoding:content-type:in-reply-to:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:from:to:cc:subject:date:message-id:reply-to :content-type; bh=Qj4vV0CvvLoLzXN5EOEjZWiqSXRAO1ek/tVDgbN2tuk=; b=a3NybSZhrtamEDzTvUaYLdxAsnVOnXDvdvuo0xjeE1TqgtG0U2/5Q+SEm6aVf9wmiy YzwqoozquuWRuyhbOVMbrHFFT8YPJFhpxIEf5U6a4FWoEzxZwdnRj71RKwFymVWmYTeu 0dzmnivj2JnZOG7jrXgH8SDuSWEIeK253YOxyE/U62Zu2X/zKfqnnUIRplaGmzZ8/jGF OLtanayg6aSnQxeDozFzLKmUTbN3zzE+ND083EdNMf1ZCuMW4HS1EluBUjhQo12mU9OB Wb+7n0fsUbGoKasXFNQWQzkrCnvfyiLwP9KiqbWiN3wMWY4ySbhj5Hhs2EanvyDcKG9e oAFQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1788602331; x=1789207131; h=content-transfer-encoding:content-type:in-reply-to:from :content-language:references:cc:to:subject:user-agent:mime-version :date:message-id:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=Qj4vV0CvvLoLzXN5EOEjZWiqSXRAO1ek/tVDgbN2tuk=; b=OSr83jn/mFITLo5s/nbhcTZSMqfMk9Mm0RmczeboSQrG6hU9rYkOeku5OLP784A6zN ZtzJS2rGQVXpSA2pB+xqiBNfuJbSseAi3w+b82jRKI+SE5RNI3zfSFf4inWbNpHfHSW3 pHJ9bPMtqQhynnyaJzJPflRlsVZJXsUlALsCmCB0ABJlNof9ntTPjE9f4td0SGRZ8AHt a1TObUxph6cWwozcrCNIgkcJwSq+9WQiKCN7FIjoH8AgT7vVzIuSfoHD/HBRE0LOvaSa VRmi8Yqp0TctJGpZeW/bR1upnhy0WCPbz5kg3jGKE5U8y7RrKuqvlA3priXpxycmE6cR 9S1A== X-Forwarded-Encrypted: i=1; AKwUvBz4sQ9rBrcxPHIVJ1TM46d8BwPnD6y+Zy28bWHWgJLDXkrZI/+fKyxD6nOtC3AeEeZ3o9m/wO5MqM1K@vger.kernel.org X-Gm-Message-State: AFuF++nc9cwtOaGBEDGKEvNNpi54amvG9ftrkufcAAardGIrjODZUAIJ N37+d4dTqbakBty6gBsNEQKF+KyJaxluucvlsxy1VWZcGnXqa9WrvkhJ X-Gm-Gg: AYBFou0e6BLfbHztPnDtxjbXnQCX9JWGm0KzfN35JE2q6WjB7el9e1LW9szzIlwUOR8 jWGa5pe1mgt6q/CTV2rtXXOK7Ojj+uHA9JKaCkATWG5gMNODzxdmPG6cfOCaDKEtWQPxJ/FqgIP OV5katkv4m9JrYUWSAUnmEmq6F1rFqgxV61i2TCP04eyeF4iS/Z5TxxEAjDD/vdnFlaVdlB6n4a 3sTQoeakECA+Jwb/6Vv4n/+qgowINcaSLVQGxwc0sQtfu9F6VoYr06H5zZ6/lWIlEhoypH2WtCA y3Cd+5yIlZJhWjEYIfmJhW2MPaDQWW2sHPETNae84zNrW0/u+j/hW4O8CVmyeH9P1yCXwc96265 NLUAWksrRwYxL2K4rl5P5pm4/jN/sVtkb8+ZjnUpe1dbcRDcUE9QdqX0vN68J3Sb4F0LwNk5aQi nGXGnIlAVcOCV9uwgJ4zeD/ahCBsej008SkN9TANlGUeOdOfWDXcWelZTaSDqdYzDpgbh5DNLng KNd/14N X-Received: by 2002:a17:90b:43:b0:395:5eec:b932 with SMTP id 98e67ed59e1d1-39b26141c79mr18617204a91.11.1788602331167; Sat, 05 Sep 2026 02:58:51 -0700 (PDT) Received: from [192.168.0.104] ([115.192.217.46]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39b07b197a9sm15276472a91.0.2026.09.05.02.58.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 05 Sep 2026 02:58:50 -0700 (PDT) Message-ID: <06252b2e-532f-4359-ad8b-f8bc1f48e73f@gmail.com> Date: Sat, 5 Sep 2026 17:58:42 +0800 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio() To: Brian Foster Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, hughd@google.com, baolin.wang@linux.alibaba.com, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, djwong@kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, chengzhihao1@huawei.com, wangkefeng.wang@huawei.com, yukuai@fnnas.com, Zhang Yi References: <20260903115018.2034541-1-yi.zhang@huaweicloud.com> Content-Language: en-US From: Zhang Yi In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/5/2026 2:03 AM, Brian Foster wrote: > On Thu, Sep 03, 2026 at 07:50:18PM +0800, Zhang Yi wrote: >> From: Zhang Yi >> >> truncate_inode_partial_folio() splits a large folio so that the caller's >> truncate loop can drop the in-range sub-folios while keeping the >> out-of-range tail. The first split at the punch start edge is >> non-uniform, which leaves the sub-folio at the truncation end edge as >> large as possible, this means it may still straddle the range, holding >> both zeroed in-range and valid out-of-range data. The function then >> attempts a second split at offset + length to isolate that tail. >> >> If the second split fails the straddling sub-folio stays merged. The >> function returned true unconditionally on all exit paths of the success >> block, telling the caller it was fully handled. The caller kept its >> default end and the truncate loop truncated every sub-folio below it, >> including the merged straddler, discarding the valid out-of-range tail. >> >> For example, a 4-page order-2 folio punched from offset 0 to the middle >> of the last page: >> >> truncate_inode_pages_range() >> truncate_inode_partial_folio() # same_folio == true >> 1st split at page0 -> [p0, p1, p2-3] # non-uniform, success >> folio2 = p2-3 # straddles: p2 zeroed, p3 tail valid >> 2nd split of folio2 fails / cannot lock >> return true # BUG: caller keeps default end >> end = 3 >> loop truncates p0, p1, p2-3 # p3's valid tail is lost >> >> This became reachable after commit 7460b470a131 ("mm/truncate: use >> folio_split() in truncate operation") replaced the atomic split_folio() >> with folio_split(), whose non-uniform split can partially split a folio >> and leave the end edge merged. >> >> It has gone unnoticed because a dirty large folio normally carries the >> filesystem's private data, for example buffer_head, so >> filemap_release_folio() -> iomap_release_folio() returns false on a >> dirty folio and folio_split() aborts with -EBUSY before any split, >> leaving the straddler safely unsplit. The bug is only reachable on paths >> that produce dirty large folios without filesystem private data, and it >> was caught on the upcoming ext4 iomap buffered I/O path when no ifs is >> attached. >> >> Rework the contract so the caller is told where to stop instead of >> silently truncating the straddler: >> >> - Return true only when a split occurred, false otherwise. This >> clarifies the existing confusing return value semantics. >> >> - Add an optional out-parameter pgoff_t *end, set to the index of the >> folio that contains @lend and must be kept by the caller's loop. It >> is only written when the folio actually straddles @lend. On the >> success path it defaults to the page index of the end edge and is >> refined to folio2->index when the second split fails to isolate the >> tail. >> >> - Rename the byte-range parameters start/end to lstart/lend to avoid >> clashing with the new @end output and to separate byte offsets from >> folio indices. >> >> Callers in truncate_inode_pages_range() and shmem_undo_range() pass &end >> only when the folio straddles lend. After all, no caller discards a >> straddling folio anymore, the in-range cleanly-split sub-folios below it >> are still dropped. >> >> Suggested-by: Brian Foster >> Link: https://lore.kernel.org/linux-fsdevel/anH-WKA1coW6wtfG@bfoster/ >> Fixes: 7460b470a131 ("mm/truncate: use folio_split() in truncate operation") >> Signed-off-by: Zhang Yi >> --- > > Thanks for the patch and description. AFAICT this looks good in terms of > I think it fixes the issue and generally otherwise preserves existing > behavior (modulo the thing Joanne is poking at that I haven't grok'd). > That said... > >> mm/internal.h | 4 ++-- >> mm/shmem.c | 12 +++++------ >> mm/truncate.c | 57 ++++++++++++++++++++++++++++++--------------------- >> 3 files changed, 41 insertions(+), 32 deletions(-) >> > ... >> diff --git a/mm/truncate.c b/mm/truncate.c >> index b58ba940be47..2ebb00f6c379 100644 >> --- a/mm/truncate.c >> +++ b/mm/truncate.c >> @@ -206,15 +206,18 @@ static int folio_split_or_unmap(struct folio *folio, struct page *split_at, >> /* >> * Handle partial folios. The folio may be entirely within the >> * range if a split has raced with us. If not, we zero the part of the >> - * folio that's within the [start, end] range, and then split the folio if >> + * folio that's within the [lstart, lend] range, and then split the folio if >> * it's large. split_page_range() will discard pages which now lie beyond >> * i_size, and we rely on the caller to discard pages which lie within a >> * newly created hole. >> * >> - * Returns false if splitting failed so the caller can avoid >> - * discarding the entire folio which is stubbornly unsplit. >> + * When @end non-NULL, set to the index of the folio that contains @lend >> + * and must be kept by the caller's truncate loop. Return %true if the >> + * folio was split, %false otherwise, in which case the folio is dropped or >> + * may still straddle the range, so the caller must not discard it. >> */ >> -bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) >> +bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart, >> + loff_t lend, pgoff_t *end) > > ... I find the interface kind of confusing and I wonder if we can come > up with something cleaner. After some time rubber ducking with $LLM, > what do you think about something where we'd have this function take > lstart/lend and always return something like pstart/pend page offsets > (instead of just end)? Those page offsets would essentially refer to the > indexes for the page range fully covered by lstart/lend, after whatever > splitting occurred (or didn't). > > So in this particular example of a 4-page folio where we truncate from 0 > to the middle of the last page, pstart would refer to p0 and pend to p3 > in the successful case. This tells the caller that pages p0 thru p2 can > be punched out. > > If the second split fails as in the commit log example, then you have > pstart == p0 and pend == p2-3. The caller (truncate_inode_pages_range()) > would reflect this into start/end such that we only punch out p0 thru p1 > in that case. > > I think that means for the case where same_folio == false, the range to > walk in the caller would be defined by pstart of the first > partial_folio() call and pend of the second, so that code would have to > change a little bit. That said, I wonder if that means we could also > just get rid of the return value and let the pstart/pend values infer > whether splits occurred or not. I see you've already removed the need > for the return check in the second partial_folio() call. I haven't run > through the other users of this function though. Thoughts on something > like that? > > Brian Thank you for your suggestion. I think I've roughly understood your idea, and it does sound promising at first glance. Yes, I've already removed the tail-side check by using the return value from pend. Similarly, it seems that we should be able to eliminate the head-side checks in both truncate_inode_pages_range() and shmem_undo_range() by leveraging pstart. But I'm still not entirely sure whether we can completely drop the return value, as I need to take a closer look at the third call site of shmem_undo_range(). I haven't fully figured that part out yet, so I'd like to analyze it more carefully. Thanks, Yi. > >> { >> loff_t pos = folio_pos(folio); >> size_t size = folio_size(folio); >> @@ -222,19 +225,22 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) >> struct page *split_at, *split_at2; >> unsigned int min_order; >> >> - if (pos < start) >> - offset = start - pos; >> + if (end && pos + size > (u64)lend) >> + *end = folio->index; >> + >> + if (pos < lstart) >> + offset = lstart - pos; >> else >> offset = 0; >> - if (pos + size <= (u64)end) >> + if (pos + size <= (u64)lend) >> length = size - offset; >> else >> - length = end + 1 - pos - offset; >> + length = lend + 1 - pos - offset; >> >> folio_wait_writeback(folio); >> if (length == size) { >> truncate_inode_folio(folio->mapping, folio); >> - return true; >> + return false; >> } >> >> /* >> @@ -248,7 +254,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) >> if (folio_needs_release(folio)) >> folio_invalidate(folio, offset, length); >> if (!folio_test_large(folio)) >> - return true; >> + return false; >> >> min_order = mapping_min_folio_order(folio->mapping); >> split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE); >> @@ -259,6 +265,10 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) >> * for shmem truncate >> */ >> struct folio *folio2; >> + bool tail_isolated = true; >> + >> + if (end) >> + *end = (pos + offset + length) >> PAGE_SHIFT; >> >> if (offset + length == size) >> goto no_split; >> @@ -273,24 +283,28 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) >> if (!folio_test_large(folio2)) >> goto out; >> >> - if (!folio_trylock(folio2)) >> + if (!folio_trylock(folio2)) { >> + tail_isolated = false; >> goto out; >> + } >> >> /* make sure folio2 is large and does not change its mapping */ >> if (folio_test_large(folio2) && >> - folio2->mapping == folio->mapping) >> - folio_split_or_unmap(folio2, split_at2, min_order); >> + folio2->mapping == folio->mapping && >> + folio_split_or_unmap(folio2, split_at2, min_order)) >> + tail_isolated = false; >> >> folio_unlock(folio2); >> out: >> + if (!tail_isolated && end) >> + *end = folio2->index; >> folio_put(folio2); >> no_split: >> return true; >> } >> - if (folio_test_dirty(folio)) >> - return false; >> - truncate_inode_folio(folio->mapping, folio); >> - return true; >> + if (!folio_test_dirty(folio)) >> + truncate_inode_folio(folio->mapping, folio); >> + return false; >> } >> >> /* >> @@ -413,11 +427,9 @@ void truncate_inode_pages_range(struct address_space *mapping, >> folio = __filemap_get_folio(mapping, lstart >> PAGE_SHIFT, FGP_LOCK, 0); >> if (!IS_ERR(folio)) { >> same_folio = lend < folio_next_pos(folio); >> - if (!truncate_inode_partial_folio(folio, lstart, lend)) { >> + if (!truncate_inode_partial_folio(folio, lstart, lend, >> + same_folio ? &end : NULL)) >> start = folio_next_index(folio); >> - if (same_folio) >> - end = folio->index; >> - } >> folio_unlock(folio); >> folio_put(folio); >> folio = NULL; >> @@ -427,8 +439,7 @@ void truncate_inode_pages_range(struct address_space *mapping, >> folio = __filemap_get_folio(mapping, lend >> PAGE_SHIFT, >> FGP_LOCK, 0); >> if (!IS_ERR(folio)) { >> - if (!truncate_inode_partial_folio(folio, lstart, lend)) >> - end = folio->index; >> + truncate_inode_partial_folio(folio, lstart, lend, &end); >> folio_unlock(folio); >> folio_put(folio); >> } >> -- >> 2.52.0 >> >