Re: [PATCH] writeback: Account the number of pages written back

[Dave Chinner <david@fromorbit.com>]

On Wed, Jun 28, 2023 at 07:55:48PM +0100, Matthew Wilcox (Oracle) wrote:
> nr_to_write is a count of pages, so we need to decrease it by the number
> of pages in the folio we just wrote, not by 1.  Most callers specify
> either LONG_MAX or 1, so are unaffected, but writeback_sb_inodes()
> might end up writing 512x as many pages as it asked for.
> 
> Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/page-writeback.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 1d17fb1ec863..d3f42009bb70 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2434,6 +2434,7 @@ int write_cache_pages(struct address_space *mapping,
>  
>  		for (i = 0; i < nr_folios; i++) {
>  			struct folio *folio = fbatch.folios[i];
> +			unsigned long nr;
>  
>  			done_index = folio->index;
>  
> @@ -2471,6 +2472,7 @@ int write_cache_pages(struct address_space *mapping,
>  
>  			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
>  			error = writepage(folio, wbc, data);
> +			nr = folio_nr_pages(folio);

This should really be done before writepage() is called, right? By
the time the writepage() returns, the folio can be unlocked, the
entire write completed and the folio partially invalidated which may
try to split the folio...

Even if this can't happen (folio refcount is elevated, right?), it
makes much more sense to me to sample the size of the folio while it
is obviously locked and not going to change...

Other than that, change looks good.

-Dave.
-- 
Dave Chinner
david@fromorbit.com