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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/8/26 17:13, 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>
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David