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Thu, 17 Sep 2026 05:32:44 -0700 (PDT) Message-ID: <997ebbdf-efc9-4dc2-91ee-254c66176c32@gmail.com> Date: Thu, 17 Sep 2026 20:32:19 +0800 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/3] mm/truncate: align truncation boundaries to mapping minimum folio order To: Zi Yan , Zhang Yi 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, bfoster@redhat.com, joannelkoong@gmail.com, djwong@kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, chengzhihao1@huawei.com, wangkefeng.wang@huawei.com, yukuai@fnnas.com References: <20260916092450.654408-1-yi.zhang@huaweicloud.com> <20260916092450.654408-3-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 9/17/2026 5:14 AM, Zi Yan wrote: > On Wed Sep 16, 2026 at 5:24 AM EDT, Zhang Yi wrote: >> From: Zhang Yi >> >> When the mapping has a non-zero minimum folio order (min_order), >> folio_split() stops at min_order instead of order 0, so the sub-folio >> containing a split point stays aligned to 1 << min_order rather than to >> a single page. The original boundaries were based on page granularity, >> so either boundary could land inside the min_order chunk at its edge, >> and the truncation loop would drop that whole chunk, valid out-of-range >> tail included. >> >> For example, a 64K (order-4) folio with min_order = 2 (16K) punched from >> offset 0 to 36K: >> >> split @p0 -> [p0-p3, p4-p7, p8-p15] # non-uniform, min_order >> folio2 = p8-p15 # straddles: p8 in range, p9-p15 tail valid >> 2nd split of folio2 -> [p8-p11, p12-p15] # success >> end(old) = p9 # BUG: p9 inside [p8-p11] >> loop truncates ... p8-p11 # p9-p11's valid tail is lost >> >> It has gone unnoticed so far for two reasons. A non-zero min_order is >> only used by filesystems with a block or sector size larger than the >> page size, and those either always write back the affected range before >> punching a hole or truncating, or they carry filesystem private data on >> dirty folios (e.g. buffer_head), which makes filemap_release_folio() >> fail and folio_split() abort with -EBUSY, so the folio is never split >> and the old start/end boundaries remain valid. The bug only becomes >> reachable on paths that truncate dirty large folios without prior >> writeback and without filesystem private data, such as the upcoming ext4 >> iomap buffered I/O path. >> >> Align both start (rounded up) and end (rounded down) to the mapping >> minimum folio order so they always fall on a folio boundary. >> >> Reported-by: Joanne Koong >> Link: https://lore.kernel.org/linux-mm/CAJnrk1bQYUe6+1ryyJur5EEnZYrC+_5AYsy=OWzVRgD4202y1g@mail.gmail.com/ >> Fixes: e220917fa5077 ("mm: split a folio in minimum folio order chunks") >> Signed-off-by: Zhang Yi >> --- >> mm/truncate.c | 14 ++++++++------ >> 1 file changed, 8 insertions(+), 6 deletions(-) >> > > This should be the first patch, since Fixes is older than the one in > patch 1. > > It can adjust start and end in truncate_inode_pages_range() instead, > since shmem's min_order is always 0. > > Something like below. > > Am I missing anything? Thanks. Yeah, this makes sense to me, thanks for your suggestion. Yi. > > diff --git a/mm/truncate.c b/mm/truncate.c > index b58ba940be474..803ee61ebf624 100644 > --- a/mm/truncate.c > +++ b/mm/truncate.c > @@ -374,6 +374,7 @@ void truncate_inode_pages_range(struct address_space *mapping, > int i; > struct folio *folio; > bool same_folio; > + pgoff_t min_nrpages = mapping_min_folio_nrpages(mapping); > > if (mapping_empty(mapping)) > return; > @@ -395,6 +396,10 @@ void truncate_inode_pages_range(struct address_space *mapping, > else > end = (lend + 1) >> PAGE_SHIFT; > > + start = round_up(start, min_nrpages); > + if (end != (pgoff_t)-1) > + end = round_down(end, min_nrpages); > + > folio_batch_init(&fbatch); > index = start; > while (index < end && find_lock_entries(mapping, &index, end - 1, >