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

[Pedro Falcato <pfalcato@suse.de>]

On Fri, Jul 03, 2026 at 10:53:24AM +0800, Lance Yang wrote:
> 
> 
> On 2026/7/3 01:24, Zi Yan wrote:
> > On 2 Jul 2026, at 12:54, 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.
> > > 
> > > 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>
> > > ---
> > > This patch is written against 7.1.0 (because the code no longer exists in mainline).
> > > 
> > > Zi, I kept your Tested-by, but I had to move some things around and
> > > use the invalidate lock. Please re-test if you can.
> > 
> > Tested it again on top of v6.12 (the patch applied cleanly) and the issue
> > is gone. My Tested-by still holds. :)

Awesome, thanks Zi!

> 
> Since READ_ONLY_THP_FOR_FS is gone from mainline, just to confirm: does this
> only affect stable kernels, right?

Correct, current mainline doesn't play any sort of fun games with
truncation and the code is fully gone from do_dentry_open().

-- 
Pedro