Re: [PATCH stable v2] mm/khugepaged: write all dirty file folios when collapsing

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 7/9/26 12:28 AM, Pedro Falcato wrote:
> Seems that I accidentally dropped linked list Cc's here, see
> https://lore.kernel.org/stable/20260708151357.353173-1-pfalcato@suse.de/
> 
> On Wed, Jul 08, 2026 at 04:13:57PM +0100, Pedro Falcato wrote:
>> [There is no upstream commit, as this code was removed by upstream
>>   commit 044925f9b565 ("mm: fs: remove filemap_nr_thps*() functions and their users")]
>>
>> As-is, khugepaged and writable-file opening exclude each other. A file
>> cannot be open writeable and have THPs (because the filesystem is not aware
>> of them). khugepaged will never collapse file pages for files that are
>> opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that
>> particular file is dropped. This is fine because nothing could've been
>> dirtied.
>>
>> However, there is an edge-case: collapse_file() might not be able to
>> coexist with concurrent writers, but it can coexist with dirty folios
>> (from previous writers). Therefore, the following can happen:
>>
>> open(file, O_RDWR)
>> write(file)
>> close(file)
>> madvise(file_mapping, MADV_COLLAPSE, some non-dirty range)
>> open(file, O_RDWR)
>>   nr_thps > 0
>>    truncate_inode_pages()
>>      /* THPs are cleared out, but so are the dirty folios */
>>
>> When this edge-case happens, there is data loss, as the dirty folios are
>> fully discarded.
>>
>> Fix it by fully writing back the page cache (and waiting) when collapsing
>> file THPs. Doing so provides the guarantee that no dirty folio will be
>> observed while there are active THPs. To fully ensure this is safe, the
>> invalidate_lock needs to be held while doing the writeout, so that
>> do_dentry_open()'s page cache truncation excludes this write-and-wait.
>>
>> As a side effect, move the nr_thps counter bumping outside the i_pages
>> lock. This is correct since the counter itself is an atomic_t and the
>> producer <-> consumer correctness is provided by a full memory barrier:
>> smp_mb() in collapse_file()/memory barrier implied by full ordering in
>> get_write_access() -> atomic_inc_unless_negative().
>>
>> Cc: stable@vger.kernel.org
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: Christian Brauner <brauner@kernel.org>
>> Cc: Jan Kara <jack@suse.cz>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Song Liu <song@kernel.org>
>> Cc: Eric Hagberg <ehagberg@janestreet.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>> Reported-by: Gregg Leventhal <gleventhal@janestreet.com>
>> Closes: https://lore.kernel.org/linux-mm/CAFN_u7H_0ECF3jixP=T=U7AH5=Q3wQNvJMo8an3VqUDMerQfUw@mail.gmail.com/
>> Tested-by: Zi Yan <ziy@nvidia.com>
>> Tested-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Pedro Falcato <pfalcato@suse.de>
>> ---

LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>