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

[Lance Yang <lance.yang@linux.dev>]

On Thu, Jul 02, 2026 at 05:54:09PM +0100, Pedro Falcato wrote:
>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.

Well spotted, thanks!

>
>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.
>
>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>
>Signed-off-by: Pedro Falcato <pfalcato@suse.de>
>---

Tested on v7.1.2. I no longer see the data loss with this patch applied.

Tested-by: Lance Yang <lance.yang@linux.dev>