Re: [PATCH 1/3] filemap: add a per-mapping stable writes flag

[Matthew Wilcox <willy@infradead.org>]

On Tue, Oct 24, 2023 at 08:44:14AM +0200, Christoph Hellwig wrote:
> folio_wait_stable waits for writeback to finish before modifying the
> contents of a folio again, e.g. to support check summing of the data
> in the block integrity code.
> 
> Currently this behavior is controlled by the SB_I_STABLE_WRITES flag
> on the super_block, which means it is uniform for the entire file system.
> This is wrong for the block device pseudofs which is shared by all
> block devices, or file systems that can use multiple devices like XFS
> witht the RT subvolume or btrfs (although btrfs currently reimplements
> folio_wait_stable anyway).
> 
> Add a per-address_space AS_STABLE_WRITES flag to control the behavior
> in a more fine grained way.  The existing SB_I_STABLE_WRITES is kept
> to initialize AS_STABLE_WRITES to the existing default which covers
> most cases.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

> +++ b/mm/page-writeback.c
> @@ -3110,7 +3110,7 @@ EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
>   */
>  void folio_wait_stable(struct folio *folio)
>  {
> -	if (folio_inode(folio)->i_sb->s_iflags & SB_I_STABLE_WRITES)
> +	if (mapping_stable_writes(folio_mapping(folio)))
>  		folio_wait_writeback(folio);

What I really like about this is that we've gone from

	folio->mapping->host->i_sb->s_iflags
to
	folio->mapping->flags

which saves us two pointer dereferences.  Sure, probably cached, but
maybe not, and cache misses are expensive.