Re: [PATCH v4 1/4] mm: track DONTCACHE dirty pages per bdi_writeback

[Jan Kara]

On Fri 01-05-26 10:49:35, Jeff Layton wrote:
> Add a per-wb WB_DONTCACHE_DIRTY counter that tracks the number of dirty
> pages with the dropbehind flag set (i.e., pages dirtied via RWF_DONTCACHE
> writes).
> 
> Increment the counter alongside WB_RECLAIMABLE in folio_account_dirtied()
> when the folio has the dropbehind flag set, and decrement it in
> folio_clear_dirty_for_io() and folio_account_cleaned(). Also decrement it
> when a non-DONTCACHE lookup clears the dropbehind flag on a dirty folio in
> __filemap_get_folio_mpol(), using proper writeback domain locking.
> 
> The counter will be used by the writeback flusher to determine how many
> pages to write back when expediting writeback for IOCB_DONTCACHE writes,
> without flushing the entire BDI's dirty pages.
> 
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Assisted-by: Claude:claude-opus-4-6
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  include/linux/backing-dev-defs.h |  1 +
>  mm/filemap.c                     | 13 ++++++++++++-
>  mm/page-writeback.c              |  6 ++++++
>  3 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index a06b93446d10..cb660dd37286 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -33,6 +33,7 @@ enum wb_stat_item {
>  	WB_WRITEBACK,
>  	WB_DIRTIED,
>  	WB_WRITTEN,
> +	WB_DONTCACHE_DIRTY,
>  	NR_WB_STAT_ITEMS
>  };
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 4e636647100c..1c9c0d5f495f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2052,8 +2052,19 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
>  	if (!folio)
>  		return ERR_PTR(-ENOENT);
>  	/* not an uncached lookup, clear uncached if set */
> -	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
> +	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE)) {
> +		if (folio_test_dirty(folio)) {
> +			struct inode *inode = mapping->host;
> +			struct bdi_writeback *wb;
> +			struct wb_lock_cookie cookie = {};
> +
> +			wb = unlocked_inode_to_wb_begin(inode, &cookie);
> +			wb_stat_mod(wb, WB_DONTCACHE_DIRTY,
> +				    -folio_nr_pages(folio));
> +			unlocked_inode_to_wb_end(inode, &cookie);
> +		}
>  		folio_clear_dropbehind(folio);
> +	}
>  	return folio;
>  }
>  EXPORT_SYMBOL(__filemap_get_folio_mpol);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 88cd53d4ba09..8e520717d1f6 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2630,6 +2630,8 @@ static void folio_account_dirtied(struct folio *folio,
>  		wb = inode_to_wb(inode);
>  
>  		lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, nr);
> +		if (folio_test_dropbehind(folio))
> +			wb_stat_mod(wb, WB_DONTCACHE_DIRTY, nr);
>  		__zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
>  		__node_stat_mod_folio(folio, NR_DIRTIED, nr);
>  		wb_stat_mod(wb, WB_RECLAIMABLE, nr);
> @@ -2651,6 +2653,8 @@ void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
>  	long nr = folio_nr_pages(folio);
>  
>  	lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
> +	if (folio_test_dropbehind(folio))
> +		wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
>  	zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
>  	wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
>  	task_io_account_cancelled_write(nr * PAGE_SIZE);
> @@ -2920,6 +2924,8 @@ bool folio_clear_dirty_for_io(struct folio *folio)
>  		if (folio_test_clear_dirty(folio)) {
>  			long nr = folio_nr_pages(folio);
>  			lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
> +			if (folio_test_dropbehind(folio))
> +				wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
>  			zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
>  			wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
>  			ret = true;
> 
> -- 
> 2.54.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR