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Sun, 06 Sep 2026 20:27:06 -0700 (PDT) Message-ID: <9974bf2a-606b-432d-95ac-2f05f3fa9953@gmail.com> Date: Mon, 7 Sep 2026 11:26:49 +0800 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: [RFC PATCH] mm/truncate: fix data loss when splitting fails in truncate_inode_partial_folio() To: Zi Yan , linux-mm@kvack.org Cc: 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, bfoster@redhat.com, djwong@kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, chengzhihao1@huawei.com, wangkefeng.wang@huawei.com, yukuai@fnnas.com, Joanne Koong , Zhang Yi References: <20260903115018.2034541-1-yi.zhang@huaweicloud.com> <4e166a3d-9012-45ab-a011-0e0ad54143d5@gmail.com> Content-Language: en-US From: Zhang Yi In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/5/2026 8:41 PM, Zi Yan wrote: > On Sat Sep 5, 2026 at 5:15 AM EDT, Zhang Yi wrote: >> On 9/5/2026 3:31 AM, Zi Yan wrote: >>> On Thu Sep 3, 2026 at 7:50 AM EDT, 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. >>> >>> Thank you for the analysis. >>> >>>> >>>> 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. >>> >>> Should we do "return false" for not split case as a minmal fix first? >>> >>> Something like below. A second patch can optimize on top of it. Let me >>> know if I miss anything. >>> >>> BTW, Claude also mentioned that if min_order > 0 and end is not aligned >>> to 1UL << min_order, there could be some issue. So >>> >>> ret = !folio_split_or_unmap(folio2, split_at2, min_order); >>> >>> should be >>> >>> unsigned long idx2 = PAGE_ALIGN_DOWN(offset + length) / PAGE_SIZE; >>> >>> ret = !folio_split_or_unmap(folio2, split_at2, min_order) && >>> IS_ALIGNED(idx2, 1UL << min_order); >>> >>> ? >>> >> >> Hi Zi Yan, >> >> Thanks for the minimal fix. I agree with the core observation — the >> tail is only really isolated when the second split and the boundary >> alignment both cooperate. However, I'd like to point out a trade-off: >> the "return false to make the caller skip the folio" mechanism leads >> to an incorrect 'start' in truncate_inode_pages_range() and over-keeps >> the in-range sub-folios. >> >> The problem is that the false return value was designed for the case >> where the folio is unsplit. Look at the caller: >> >> if (!truncate_inode_partial_folio(folio, lstart, lend)) { >> start = folio_next_index(folio); >> if (same_folio) >> end = folio->index; >> } >> >> start = folio_next_index(folio); end = folio->index only makes sense >> when folio is still the whole folio. But after the first split >> succeeds, the caller's folio reference has already been transferred to >> the sub-folio containing split_at, so folio is no longer the whole >> folio. >> >> Concretely, take the commit's example: a 4-page order-2 folio >> [p0 p1 p2 p3], punched from offset 0 into the middle of p3, >> min_order == 0: >> >> [p0 p1 p2 p3] --1st split @p0--> [p0] [p1] [p2-p3] >> folio now points to [p0] >> folio2 = [p2-p3] # p2 zeroed, p3 tail valid >> 2nd split of [p2-p3] fails # e.g. -EBUSY, or can't lock >> >> With your fix, ret becomes false, so the caller runs: >> >> start = folio_next_index([p0]) = 1; >> end = folio->index = 0; >> >> The truncate loop then does while (index < end) -> 1 < 0 -> nothing, >> and [p0] and [p1] are left in the page cache, even though they are >> fully inside the punched range and should have been dropped. >> >> To be fair, this will not lead to any data-corruption problem because >> p0 and p1 are already zeroed. So as a minimal fix to stop the data >> loss, it is acceptable. But it still leaves the in-range sub-folios >> behind and wastes memory, which somewhat reduces the benefit of >> splitting the folio. So I don't think change the return value alone can >> solve this problem. >> >> What do you think? >> > > Understood. > > Alternatives are changing folio_split() to make sure folio2 is split. > From weakest guarantee to strongest guarantee: > > 1. make folio_split() return with folio2 locked: but others can still > put a ref on folio2 to fail the subsequent folio2 split. > > 2. make folio_split() accept two split_at, folio_split() does both folio > and folio2 split internally: but if folio2 split require an xa_node and > the allocation fails, folio2 split can still fail. > > 3. make folio_split() accept two split_at and preallocate two xa_node > upfront: this should guarantee folio_split() to either split both folio > and folio2 or split nothing, but it specializes folio_split() for > truncate. > I'm afraid even guaranteeing a successful folio2 split is not enough here. As Joanne pointed out [1], when min_order is not 0, even if folio2 is successfully split and returns true, the folio remains a large folio. In the outer loop, 'end' still cannot point to the head of the split folio2, so the entire folio is still incorrectly dropped as a whole, losing the valid data at the tail. [1] https://lore.kernel.org/linux-mm/CAJnrk1bQYUe6+1ryyJur5EEnZYrC+_5AYsy=OWzVRgD4202y1g@mail.gmail.com/ > I guess for now it might be better to make truncate to handle the > folio2-not-split situation. > Yeah, agree. Thanks, Yi.