Re: [PATCH v2 1/1] fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes()

[Bernd Schubert <bernd@bsbernd.com>]

On 12/15/25 04:00, Joanne Koong wrote:
> Skip waiting on writeback for inodes that belong to mappings that do not
> have data integrity guarantees (denoted by the AS_NO_DATA_INTEGRITY
> mapping flag).
> 
> This restores fuse back to prior behavior where syncs are no-ops. This
> is needed because otherwise, if a system is running a faulty fuse
> server that does not reply to issued write requests, this will cause
> wait_sb_inodes() to wait forever.
> 
> Fixes: 0c58a97f919c ("fuse: remove tmp folio for writebacks and internal rb tree")
> Reported-by: Athul Krishna <athul.krishna.kr@protonmail.com>
> Reported-by: J. Neuschäfer <j.neuschaefer@gmx.net>
> Cc: stable@vger.kernel.org
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>   fs/fs-writeback.c       |  3 ++-
>   fs/fuse/file.c          |  4 +++-
>   include/linux/pagemap.h | 11 +++++++++++
>   3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 6800886c4d10..ab2e279ed3c2 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -2751,7 +2751,8 @@ static void wait_sb_inodes(struct super_block *sb)
>   		 * do not have the mapping lock. Skip it here, wb completion
>   		 * will remove it.
>   		 */
> -		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
> +		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK) ||
> +		    mapping_no_data_integrity(mapping))
>   			continue;
>   
>   		spin_unlock_irq(&sb->s_inode_wblist_lock);
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 01bc894e9c2b..3b2a171e652f 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -3200,8 +3200,10 @@ void fuse_init_file_inode(struct inode *inode, unsigned int flags)
>   
>   	inode->i_fop = &fuse_file_operations;
>   	inode->i_data.a_ops = &fuse_file_aops;
> -	if (fc->writeback_cache)
> +	if (fc->writeback_cache) {
>   		mapping_set_writeback_may_deadlock_on_reclaim(&inode->i_data);
> +		mapping_set_no_data_integrity(&inode->i_data);
> +	}

For a future commit, maybe we could add a FUSE_INIT flag that allows privileged
fuse server to not set this? Maybe even in combination with an enforced request
timeout?

>   
>   	INIT_LIST_HEAD(&fi->write_files);
>   	INIT_LIST_HEAD(&fi->queued_writes);
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 31a848485ad9..ec442af3f886 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -210,6 +210,7 @@ enum mapping_flags {
>   	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
>   	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
>   				   account usage to user cgroups */
> +	AS_NO_DATA_INTEGRITY = 11, /* no data integrity guarantees */
>   	/* Bits 16-25 are used for FOLIO_ORDER */
>   	AS_FOLIO_ORDER_BITS = 5,
>   	AS_FOLIO_ORDER_MIN = 16,
> @@ -345,6 +346,16 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct addres
>   	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>   }
>   
> +static inline void mapping_set_no_data_integrity(struct address_space *mapping)
> +{
> +	set_bit(AS_NO_DATA_INTEGRITY, &mapping->flags);
> +}
> +
> +static inline bool mapping_no_data_integrity(const struct address_space *mapping)
> +{
> +	return test_bit(AS_NO_DATA_INTEGRITY, &mapping->flags);
> +}
> +
>   static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)
>   {
>   	return mapping->gfp_mask;


Reviewed-by: Bernd Schubert <bschubert@ddn.com>