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

[Pedro Falcato <pfalcato@suse.de>]

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>
> ---
> v2:
>  - condition this logic on !mapping_large_folio_support(mapping) (Baolin, Lance, Matthew)
>  - explain why moving the nr_thps bumping outside the i_pages lock is safe (Matthew)
>  - pick up Tested-by from Lance (thank you!)
> 
>  mm/khugepaged.c | 39 +++++++++++++++++++++++++--------------
>  1 file changed, 25 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b8452dbdb043..d6e04041f5dc 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2094,32 +2094,43 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  		goto xa_unlocked;
>  	}
>  
> -	if (!is_shmem) {
> +xa_locked:
> +	xas_unlock_irq(&xas);
> +xa_unlocked:
> +
> +	/*
> +	 * If collapse is successful, flush must be done now before copying.
> +	 * If collapse is unsuccessful, does flush actually need to be done?
> +	 * Do it anyway, to clear the state.
> +	 */
> +	try_to_unmap_flush();
> +
> +	if (result == SCAN_SUCCEED && !is_shmem && !mapping_large_folio_support(mapping)) {
> +		/*
> +		 * invalidate_lock as shared excludes against concurrent opens
> +		 * in do_dentry_open() truncating the page cache. This is
> +		 * particularly important if there are dirty folios in transit.
> +		 */
> +		filemap_invalidate_lock_shared(mapping);
>  		filemap_nr_thps_inc(mapping);
>  		/*
>  		 * Paired with the fence in do_dentry_open() -> get_write_access()
>  		 * to ensure i_writecount is up to date and the update to nr_thps
>  		 * is visible. Ensures the page cache will be truncated if the
> -		 * file is opened writable.
> +		 * file is opened writable. If collapse looks to be successful,
> +		 * flush any dirty pages out the page cache. With the nr_thps
> +		 * incremented, there won't be any new writers (nor new dirties).
>  		 */
>  		smp_mb();
> -		if (inode_is_open_for_write(mapping->host)) {
> +		if (inode_is_open_for_write(mapping->host) || filemap_write_and_wait(mapping)) {
>  			result = SCAN_FAIL;
>  			filemap_nr_thps_dec(mapping);
> +			filemap_invalidate_unlock_shared(mapping);
> +			goto rollback;
>  		}
> +		filemap_invalidate_unlock_shared(mapping);
>  	}
>  
> -xa_locked:
> -	xas_unlock_irq(&xas);
> -xa_unlocked:
> -
> -	/*
> -	 * If collapse is successful, flush must be done now before copying.
> -	 * If collapse is unsuccessful, does flush actually need to be done?
> -	 * Do it anyway, to clear the state.
> -	 */
> -	try_to_unmap_flush();
> -
>  	if (result == SCAN_SUCCEED && nr_none &&
>  	    !shmem_charge(mapping->host, nr_none))
>  		result = SCAN_FAIL;
> -- 
> 2.54.0
> 

-- 
Pedro