Re: [PATCH v2 3/3] iomap: clean up writeback state logic on writepage error

[Allison Henderson <allison.henderson@oracle.com>]

On 10/29/20 6:23 AM, Brian Foster wrote:
> The iomap writepage error handling logic is a mash of old and
> slightly broken XFS writepage logic. When keepwrite writeback state
> tracking was introduced in XFS in commit 0d085a529b42 ("xfs: ensure
> WB_SYNC_ALL writeback handles partial pages correctly"), XFS had an
> additional cluster writeback context that scanned ahead of
> ->writepage() to process dirty pages over the current ->writepage()
> extent mapping. This context expected a dirty page and required
> retention of the TOWRITE tag on partial page processing so the
> higher level writeback context would revisit the page (in contrast
> to ->writepage(), which passes a page with the dirty bit already
> cleared).
> 
> The cluster writeback mechanism was eventually removed and some of
> the error handling logic folded into the primary writeback path in
> commit 150d5be09ce4 ("xfs: remove xfs_cancel_ioend"). This patch
> accidentally conflated the two contexts by using the keepwrite logic
> in ->writepage() without accounting for the fact that the page is
> not dirty. Further, the keepwrite logic has no practical effect on
> the core ->writepage() caller (write_cache_pages()) because it never
> revisits a page in the current function invocation.
> 
> Technically, the page should be redirtied for the keepwrite logic to
> have any effect. Otherwise, write_cache_pages() may find the tagged
> page but will skip it since it is clean. Even if the page was
> redirtied, however, there is still no practical effect to keepwrite
> since write_cache_pages() does not wrap around within a single
> invocation of the function. Therefore, the dirty page would simply
> end up retagged on the next writeback sequence over the associated
> range.
> 
> All that being said, none of this really matters because redirtying
> a partially processed page introduces a potential infinite redirty
> -> writeback failure loop that deviates from the current design
> principle of clearing the dirty state on writepage failure to avoid
> building up too much dirty, unreclaimable memory on the system.
> Therefore, drop the spurious keepwrite usage and dirty state
> clearing logic from iomap_writepage_map(), treat the partially
> processed page the same as a fully processed page, and let the
> imminent ioend failure clean up the writeback state.
> 
Ok, thanks for all the explaining.  Makes sense :-)
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>   fs/iomap/buffered-io.c | 15 ++-------------
>   1 file changed, 2 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d1f04eabc7e4..e3a4568f6c2e 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1404,6 +1404,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>   	WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
>   	WARN_ON_ONCE(!PageLocked(page));
>   	WARN_ON_ONCE(PageWriteback(page));
> +	WARN_ON_ONCE(PageDirty(page));
>   
>   	/*
>   	 * We cannot cancel the ioend directly here on error.  We may have
> @@ -1425,21 +1426,9 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>   			unlock_page(page);
>   			goto done;
>   		}
> -
> -		/*
> -		 * If the page was not fully cleaned, we need to ensure that the
> -		 * higher layers come back to it correctly.  That means we need
> -		 * to keep the page dirty, and for WB_SYNC_ALL writeback we need
> -		 * to ensure the PAGECACHE_TAG_TOWRITE index mark is not removed
> -		 * so another attempt to write this page in this writeback sweep
> -		 * will be made.
> -		 */
> -		set_page_writeback_keepwrite(page);
> -	} else {
> -		clear_page_dirty_for_io(page);
> -		set_page_writeback(page);
>   	}
>   
> +	set_page_writeback(page);
>   	unlock_page(page);
>   
>   	/*
>