[PATCH v3] xfs: don't eat an EIO/ENOSPC writeback error when scrubbing data fork

[PATCH v3] xfs: don't eat an EIO/ENOSPC writeback error when scrubbing data fork

[Darrick J. Wong]

From: Darrick J. Wong <darrick.wong@oracle.com>

The data fork scrubber calls filemap_write_and_wait to flush dirty pages
and delalloc reservations out to disk prior to checking the data fork's
extent mappings.  Unfortunately, this means that scrub can consume the
EIO/ENOSPC errors that would otherwise have stayed around in the address
space until (we hope) the writer application calls fsync to persist data
and collect errors.  The end result is that programs that wrote to a
file might never see the error code and proceed as if nothing were
wrong.

xfs_scrub is not in a position to notify file writers about the
writeback failure, and it's only here to check metadata, not file
contents.  Therefore, if writeback fails, we should stuff the error code
back into the address space so that an fsync by the writer application
can pick that up.

Fixes: 99d9d8d05da2 ("xfs: scrub inode block mappings")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v3: don't play this game where we clear the mapping error only to re-set it
v2: explain why it's ok to keep going even if writeback fails
---
 fs/xfs/scrub/bmap.c |   25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/scrub/bmap.c b/fs/xfs/scrub/bmap.c
index 7badd6dfe544..c4c2477a94ae 100644
--- a/fs/xfs/scrub/bmap.c
+++ b/fs/xfs/scrub/bmap.c
@@ -45,9 +45,30 @@ xchk_setup_inode_bmap(
 	 */
 	if (S_ISREG(VFS_I(sc->ip)->i_mode) &&
 	    sc->sm->sm_type == XFS_SCRUB_TYPE_BMBTD) {
+		struct address_space	*mapping = VFS_I(sc->ip)->i_mapping;
+
 		inode_dio_wait(VFS_I(sc->ip));
-		error = filemap_write_and_wait(VFS_I(sc->ip)->i_mapping);
-		if (error)
+
+		/*
+		 * Try to flush all incore state to disk before we examine the
+		 * space mappings for the data fork.  Leave accumulated errors
+		 * in the mapping for the writer threads to consume.
+		 */
+		error = filemap_fdatawrite(mapping);
+		if (!error)
+			error = filemap_fdatawait_keep_errors(mapping);
+		if (error == -ENOSPC || error == -EIO) {
+			/*
+			 * On ENOSPC or EIO writeback errors, we continue into
+			 * the extent mapping checks because write failures do
+			 * not necessarily imply anything about the correctness
+			 * of the file metadata.  The metadata and the file
+			 * data could be on completely separate devices; a
+			 * media failure might only affect a subset of the
+			 * disk, etc.  We can handle delalloc extents in the
+			 * scrubber, so leaving them in memory is fine.
+			 */
+		} else if (error)
 			goto out;
 	}
 

Re: [PATCH v3] xfs: don't eat an EIO/ENOSPC writeback error when scrubbing data fork

[Dave Chinner]

On Tue, Jun 23, 2020 at 08:51:33PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> The data fork scrubber calls filemap_write_and_wait to flush dirty pages
> and delalloc reservations out to disk prior to checking the data fork's
> extent mappings.  Unfortunately, this means that scrub can consume the
> EIO/ENOSPC errors that would otherwise have stayed around in the address
> space until (we hope) the writer application calls fsync to persist data
> and collect errors.  The end result is that programs that wrote to a
> file might never see the error code and proceed as if nothing were
> wrong.
> 
> xfs_scrub is not in a position to notify file writers about the
> writeback failure, and it's only here to check metadata, not file
> contents.  Therefore, if writeback fails, we should stuff the error code
> back into the address space so that an fsync by the writer application
> can pick that up.
> 
> Fixes: 99d9d8d05da2 ("xfs: scrub inode block mappings")
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v3: don't play this game where we clear the mapping error only to re-set it
> v2: explain why it's ok to keep going even if writeback fails
> ---
>  fs/xfs/scrub/bmap.c |   25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/scrub/bmap.c b/fs/xfs/scrub/bmap.c
> index 7badd6dfe544..c4c2477a94ae 100644
> --- a/fs/xfs/scrub/bmap.c
> +++ b/fs/xfs/scrub/bmap.c
> @@ -45,9 +45,30 @@ xchk_setup_inode_bmap(
>  	 */
>  	if (S_ISREG(VFS_I(sc->ip)->i_mode) &&
>  	    sc->sm->sm_type == XFS_SCRUB_TYPE_BMBTD) {
> +		struct address_space	*mapping = VFS_I(sc->ip)->i_mapping;
> +
>  		inode_dio_wait(VFS_I(sc->ip));
> -		error = filemap_write_and_wait(VFS_I(sc->ip)->i_mapping);
> -		if (error)
> +
> +		/*
> +		 * Try to flush all incore state to disk before we examine the
> +		 * space mappings for the data fork.  Leave accumulated errors
> +		 * in the mapping for the writer threads to consume.
> +		 */
> +		error = filemap_fdatawrite(mapping);
> +		if (!error)
> +			error = filemap_fdatawait_keep_errors(mapping);
> +		if (error == -ENOSPC || error == -EIO) {
> +			/*
> +			 * On ENOSPC or EIO writeback errors, we continue into
> +			 * the extent mapping checks because write failures do
> +			 * not necessarily imply anything about the correctness
> +			 * of the file metadata.  The metadata and the file
> +			 * data could be on completely separate devices; a
> +			 * media failure might only affect a subset of the
> +			 * disk, etc.  We can handle delalloc extents in the
> +			 * scrubber, so leaving them in memory is fine.
> +			 */
> +		} else if (error)
>  			goto out;
>  	}

Functionality is fine, but the if() that does nothing but contain a
comment is a bit weird. Promote the error handling comment, and that
can formatted as:

		/*
		 * Try to flush all incore state to disk before we examine the
		 * space mappings for the data fork.  Leave accumulated errors
		 * in the mapping for the writer threads to consume.
		 *
		 * On ENOSPC or EIO writeback errors, we continue into the
		 * extent mapping checks because write failures do not
		 * necessarily imply anything about the correctness of the file
		 * metadata.  The metadata and the file data could be on
		 * completely separate devices; a media failure might only
		 * affect a subset of the disk, etc.  We can handle delalloc
		 * extents in the scrubber, so leaving them in memory is fine.
		 */
		error = filemap_fdatawrite(mapping);
		if (!error)
			error = filemap_fdatawait_keep_errors(mapping);
		if (error && (error != -ENOSPC && error !== -EIO))
			goto out;

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com