* Re: [PATCH] xfs: trim writepage mapping to within eof
2017-10-12 11:47 [PATCH] xfs: trim writepage mapping to within eof Brian Foster
@ 2017-10-12 15:53 ` Eryu Guan
2017-10-12 20:05 ` Darrick J. Wong
2017-10-12 21:22 ` Dave Chinner
2 siblings, 0 replies; 6+ messages in thread
From: Eryu Guan @ 2017-10-12 15:53 UTC (permalink / raw)
To: Brian Foster; +Cc: linux-xfs
On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> The writeback rework in commit fbcc02561359 ("xfs: Introduce
> writeback context for writepages") introduced a subtle change in
> behavior with regard to the block mapping used across the
> ->writepages() sequence. The previous xfs_cluster_write() code would
> only flush pages up to EOF at the time of the writepage, thus
> ensuring that any pages due to file-extending writes would be
> handled on a separate cycle and with a new, updated block mapping.
>
> The updated code establishes a block mapping in xfs_writepage_map()
> that could extend beyond EOF if the file has post-eof preallocation.
> Because we now use the generic writeback infrastructure and pass the
> cached mapping to each writepage call, there is no implicit EOF
> limit in place. If eofblocks trimming occurs during ->writepages(),
> any post-eof portion of the cached mapping becomes invalid. The
> eofblocks code has no means to serialize against writeback because
> there are no pages associated with post-eof blocks. Therefore if an
> eofblocks trim occurs and is followed by a file-extending buffered
> write, not only has the mapping become invalid, but we could end up
> writing a page to disk based on the invalid mapping.
>
> Consider the following sequence of events:
>
> - A buffered write creates a delalloc extent and post-eof
> speculative preallocation.
> - Writeback starts and on the first writepage cycle, the delalloc
> extent is converted to real blocks (including the post-eof blocks)
> and the mapping is cached.
> - The file is closed and xfs_release() trims post-eof blocks. The
> cached writeback mapping is now invalid.
> - Another buffered write appends the file with a delalloc extent.
> - The concurrent writeback cycle picks up the just written page
> because the writeback range end is LLONG_MAX. xfs_writepage_map()
> attributes it to the (now invalid) cached mapping and writes the
> data to an incorrect location on disk (and where the file offset is
> still backed by a delalloc extent).
>
> This problem is reproduced by xfstests test generic/463, which
> triggers racing writes, appends, open/closes and writeback requests.
Most probably the test seq number will be generic/464, I renumbered at
commit time. I'll push it out this week. Just FYI.
>
> To address this problem, trim the mapping used during writeback to
> within EOF when the mapping is created. This ensures the mapping is
> revalidated for any pages encountered beyond EOF as of the time the
> current mapping was cached.
>
> Reported-by: Eryu Guan <eguan@redhat.com>
> Diagnosed-by: Eryu Guan <eguan@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>
> Hi all,
>
> This is a followup to the issue Eryu tracked down, described here[1].
>
> Note that this patch will not deal with any writeback mapping validity
> issues not associated with eofblocks management. Dave is working on a
> more generic approach to deal with such problems. This patch is intended
> to be a targeted and backportable fix for the regression in the
> writeback code.
>
> Brian
Thanks for the followup!
Eryu
>
> [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
>
> fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
> fs/xfs/libxfs/xfs_bmap.h | 1 +
> fs/xfs/xfs_aops.c | 6 ++++--
> 3 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 044a363..dd3fb7b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3852,6 +3852,17 @@ xfs_trim_extent(
> }
> }
>
> +/* trim extent to within eof */
> +void
> +xfs_trim_extent_eof(
> + struct xfs_bmbt_irec *irec,
> + struct xfs_inode *ip)
> +
> +{
> + xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> + i_size_read(VFS_I(ip))));
> +}
> +
> /*
> * Trim the returned map to the required bounds
> */
> diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> index 851982a..502e0d8 100644
> --- a/fs/xfs/libxfs/xfs_bmap.h
> +++ b/fs/xfs/libxfs/xfs_bmap.h
> @@ -208,6 +208,7 @@ void xfs_bmap_trace_exlist(struct xfs_inode *ip, xfs_extnum_t cnt,
>
> void xfs_trim_extent(struct xfs_bmbt_irec *irec, xfs_fileoff_t bno,
> xfs_filblks_t len);
> +void xfs_trim_extent_eof(struct xfs_bmbt_irec *, struct xfs_inode *);
> int xfs_bmap_add_attrfork(struct xfs_inode *ip, int size, int rsvd);
> void xfs_bmap_local_to_extents_empty(struct xfs_inode *ip, int whichfork);
> void xfs_bmap_add_free(struct xfs_mount *mp, struct xfs_defer_ops *dfops,
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 1dbc5cf..3ab6d9d 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -423,7 +423,7 @@ xfs_map_blocks(
> imap);
> if (!error)
> trace_xfs_map_blocks_alloc(ip, offset, count, type, imap);
> - return error;
> + goto out_trim;
> }
>
> #ifdef DEBUG
> @@ -435,7 +435,9 @@ xfs_map_blocks(
> #endif
> if (nimaps)
> trace_xfs_map_blocks_found(ip, offset, count, type, imap);
> - return 0;
> +out_trim:
> + xfs_trim_extent_eof(imap, ip);
> + return error;
> }
>
> STATIC bool
> --
> 2.9.5
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] xfs: trim writepage mapping to within eof
2017-10-12 11:47 [PATCH] xfs: trim writepage mapping to within eof Brian Foster
2017-10-12 15:53 ` Eryu Guan
@ 2017-10-12 20:05 ` Darrick J. Wong
2017-10-12 20:44 ` Brian Foster
2017-10-12 21:22 ` Dave Chinner
2 siblings, 1 reply; 6+ messages in thread
From: Darrick J. Wong @ 2017-10-12 20:05 UTC (permalink / raw)
To: Brian Foster; +Cc: linux-xfs
On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> The writeback rework in commit fbcc02561359 ("xfs: Introduce
> writeback context for writepages") introduced a subtle change in
> behavior with regard to the block mapping used across the
> ->writepages() sequence. The previous xfs_cluster_write() code would
> only flush pages up to EOF at the time of the writepage, thus
> ensuring that any pages due to file-extending writes would be
> handled on a separate cycle and with a new, updated block mapping.
>
> The updated code establishes a block mapping in xfs_writepage_map()
> that could extend beyond EOF if the file has post-eof preallocation.
> Because we now use the generic writeback infrastructure and pass the
> cached mapping to each writepage call, there is no implicit EOF
> limit in place. If eofblocks trimming occurs during ->writepages(),
> any post-eof portion of the cached mapping becomes invalid. The
> eofblocks code has no means to serialize against writeback because
> there are no pages associated with post-eof blocks. Therefore if an
> eofblocks trim occurs and is followed by a file-extending buffered
> write, not only has the mapping become invalid, but we could end up
> writing a page to disk based on the invalid mapping.
>
> Consider the following sequence of events:
>
> - A buffered write creates a delalloc extent and post-eof
> speculative preallocation.
> - Writeback starts and on the first writepage cycle, the delalloc
> extent is converted to real blocks (including the post-eof blocks)
> and the mapping is cached.
> - The file is closed and xfs_release() trims post-eof blocks. The
> cached writeback mapping is now invalid.
> - Another buffered write appends the file with a delalloc extent.
> - The concurrent writeback cycle picks up the just written page
> because the writeback range end is LLONG_MAX. xfs_writepage_map()
> attributes it to the (now invalid) cached mapping and writes the
> data to an incorrect location on disk (and where the file offset is
> still backed by a delalloc extent).
>
> This problem is reproduced by xfstests test generic/463, which
> triggers racing writes, appends, open/closes and writeback requests.
>
> To address this problem, trim the mapping used during writeback to
> within EOF when the mapping is created. This ensures the mapping is
> revalidated for any pages encountered beyond EOF as of the time the
> current mapping was cached.
>
> Reported-by: Eryu Guan <eguan@redhat.com>
> Diagnosed-by: Eryu Guan <eguan@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
Will throw this into the testing machine. Is this serious enough to
push into 4.14? I'm thinking of doing one more 4.14-fixes next week...
...granted I mostly hear my stomach churning "ye $deities another one of
these stale writeback blahblah problem workarounds" but this would seem
to fix a file corruption problem. :(
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
--D
> ---
>
> Hi all,
>
> This is a followup to the issue Eryu tracked down, described here[1].
>
> Note that this patch will not deal with any writeback mapping validity
> issues not associated with eofblocks management. Dave is working on a
> more generic approach to deal with such problems. This patch is intended
> to be a targeted and backportable fix for the regression in the
> writeback code.
>
> Brian
>
> [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
>
> fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
> fs/xfs/libxfs/xfs_bmap.h | 1 +
> fs/xfs/xfs_aops.c | 6 ++++--
> 3 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 044a363..dd3fb7b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3852,6 +3852,17 @@ xfs_trim_extent(
> }
> }
>
> +/* trim extent to within eof */
> +void
> +xfs_trim_extent_eof(
> + struct xfs_bmbt_irec *irec,
> + struct xfs_inode *ip)
> +
> +{
> + xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> + i_size_read(VFS_I(ip))));
> +}
> +
> /*
> * Trim the returned map to the required bounds
> */
> diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> index 851982a..502e0d8 100644
> --- a/fs/xfs/libxfs/xfs_bmap.h
> +++ b/fs/xfs/libxfs/xfs_bmap.h
> @@ -208,6 +208,7 @@ void xfs_bmap_trace_exlist(struct xfs_inode *ip, xfs_extnum_t cnt,
>
> void xfs_trim_extent(struct xfs_bmbt_irec *irec, xfs_fileoff_t bno,
> xfs_filblks_t len);
> +void xfs_trim_extent_eof(struct xfs_bmbt_irec *, struct xfs_inode *);
> int xfs_bmap_add_attrfork(struct xfs_inode *ip, int size, int rsvd);
> void xfs_bmap_local_to_extents_empty(struct xfs_inode *ip, int whichfork);
> void xfs_bmap_add_free(struct xfs_mount *mp, struct xfs_defer_ops *dfops,
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 1dbc5cf..3ab6d9d 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -423,7 +423,7 @@ xfs_map_blocks(
> imap);
> if (!error)
> trace_xfs_map_blocks_alloc(ip, offset, count, type, imap);
> - return error;
> + goto out_trim;
> }
>
> #ifdef DEBUG
> @@ -435,7 +435,9 @@ xfs_map_blocks(
> #endif
> if (nimaps)
> trace_xfs_map_blocks_found(ip, offset, count, type, imap);
> - return 0;
> +out_trim:
> + xfs_trim_extent_eof(imap, ip);
> + return error;
> }
>
> STATIC bool
> --
> 2.9.5
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] xfs: trim writepage mapping to within eof
2017-10-12 20:05 ` Darrick J. Wong
@ 2017-10-12 20:44 ` Brian Foster
0 siblings, 0 replies; 6+ messages in thread
From: Brian Foster @ 2017-10-12 20:44 UTC (permalink / raw)
To: Darrick J. Wong; +Cc: linux-xfs
On Thu, Oct 12, 2017 at 01:05:50PM -0700, Darrick J. Wong wrote:
> On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> > The writeback rework in commit fbcc02561359 ("xfs: Introduce
> > writeback context for writepages") introduced a subtle change in
> > behavior with regard to the block mapping used across the
> > ->writepages() sequence. The previous xfs_cluster_write() code would
> > only flush pages up to EOF at the time of the writepage, thus
> > ensuring that any pages due to file-extending writes would be
> > handled on a separate cycle and with a new, updated block mapping.
> >
> > The updated code establishes a block mapping in xfs_writepage_map()
> > that could extend beyond EOF if the file has post-eof preallocation.
> > Because we now use the generic writeback infrastructure and pass the
> > cached mapping to each writepage call, there is no implicit EOF
> > limit in place. If eofblocks trimming occurs during ->writepages(),
> > any post-eof portion of the cached mapping becomes invalid. The
> > eofblocks code has no means to serialize against writeback because
> > there are no pages associated with post-eof blocks. Therefore if an
> > eofblocks trim occurs and is followed by a file-extending buffered
> > write, not only has the mapping become invalid, but we could end up
> > writing a page to disk based on the invalid mapping.
> >
> > Consider the following sequence of events:
> >
> > - A buffered write creates a delalloc extent and post-eof
> > speculative preallocation.
> > - Writeback starts and on the first writepage cycle, the delalloc
> > extent is converted to real blocks (including the post-eof blocks)
> > and the mapping is cached.
> > - The file is closed and xfs_release() trims post-eof blocks. The
> > cached writeback mapping is now invalid.
> > - Another buffered write appends the file with a delalloc extent.
> > - The concurrent writeback cycle picks up the just written page
> > because the writeback range end is LLONG_MAX. xfs_writepage_map()
> > attributes it to the (now invalid) cached mapping and writes the
> > data to an incorrect location on disk (and where the file offset is
> > still backed by a delalloc extent).
> >
> > This problem is reproduced by xfstests test generic/463, which
> > triggers racing writes, appends, open/closes and writeback requests.
> >
> > To address this problem, trim the mapping used during writeback to
> > within EOF when the mapping is created. This ensures the mapping is
> > revalidated for any pages encountered beyond EOF as of the time the
> > current mapping was cached.
> >
> > Reported-by: Eryu Guan <eguan@redhat.com>
> > Diagnosed-by: Eryu Guan <eguan@redhat.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
>
> Will throw this into the testing machine. Is this serious enough to
> push into 4.14? I'm thinking of doing one more 4.14-fixes next week...
>
I think it's semi-serious due to, as you're aware, the side effect of
the problem. That said, it's been a regression since v4.6 or so and
detected via a stress workload as opposed to a user report, so it's
apparently not the most prevalent thing.
> ...granted I mostly hear my stomach churning "ye $deities another one of
> these stale writeback blahblah problem workarounds" but this would seem
> to fix a file corruption problem. :(
>
Heh. :P I guess if it were me I would include it so long as there is
enough time to test, but I wouldn't lose sleep if it landed in v4.15 due
to the above.
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
>
Thanks!
Brian
> --D
>
> > ---
> >
> > Hi all,
> >
> > This is a followup to the issue Eryu tracked down, described here[1].
> >
> > Note that this patch will not deal with any writeback mapping validity
> > issues not associated with eofblocks management. Dave is working on a
> > more generic approach to deal with such problems. This patch is intended
> > to be a targeted and backportable fix for the regression in the
> > writeback code.
> >
> > Brian
> >
> > [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
> >
> > fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
> > fs/xfs/libxfs/xfs_bmap.h | 1 +
> > fs/xfs/xfs_aops.c | 6 ++++--
> > 3 files changed, 16 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 044a363..dd3fb7b 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -3852,6 +3852,17 @@ xfs_trim_extent(
> > }
> > }
> >
> > +/* trim extent to within eof */
> > +void
> > +xfs_trim_extent_eof(
> > + struct xfs_bmbt_irec *irec,
> > + struct xfs_inode *ip)
> > +
> > +{
> > + xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> > + i_size_read(VFS_I(ip))));
> > +}
> > +
> > /*
> > * Trim the returned map to the required bounds
> > */
> > diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
> > index 851982a..502e0d8 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.h
> > +++ b/fs/xfs/libxfs/xfs_bmap.h
> > @@ -208,6 +208,7 @@ void xfs_bmap_trace_exlist(struct xfs_inode *ip, xfs_extnum_t cnt,
> >
> > void xfs_trim_extent(struct xfs_bmbt_irec *irec, xfs_fileoff_t bno,
> > xfs_filblks_t len);
> > +void xfs_trim_extent_eof(struct xfs_bmbt_irec *, struct xfs_inode *);
> > int xfs_bmap_add_attrfork(struct xfs_inode *ip, int size, int rsvd);
> > void xfs_bmap_local_to_extents_empty(struct xfs_inode *ip, int whichfork);
> > void xfs_bmap_add_free(struct xfs_mount *mp, struct xfs_defer_ops *dfops,
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 1dbc5cf..3ab6d9d 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -423,7 +423,7 @@ xfs_map_blocks(
> > imap);
> > if (!error)
> > trace_xfs_map_blocks_alloc(ip, offset, count, type, imap);
> > - return error;
> > + goto out_trim;
> > }
> >
> > #ifdef DEBUG
> > @@ -435,7 +435,9 @@ xfs_map_blocks(
> > #endif
> > if (nimaps)
> > trace_xfs_map_blocks_found(ip, offset, count, type, imap);
> > - return 0;
> > +out_trim:
> > + xfs_trim_extent_eof(imap, ip);
> > + return error;
> > }
> >
> > STATIC bool
> > --
> > 2.9.5
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] xfs: trim writepage mapping to within eof
2017-10-12 11:47 [PATCH] xfs: trim writepage mapping to within eof Brian Foster
2017-10-12 15:53 ` Eryu Guan
2017-10-12 20:05 ` Darrick J. Wong
@ 2017-10-12 21:22 ` Dave Chinner
2017-10-13 11:42 ` Brian Foster
2 siblings, 1 reply; 6+ messages in thread
From: Dave Chinner @ 2017-10-12 21:22 UTC (permalink / raw)
To: Brian Foster; +Cc: linux-xfs
On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> The writeback rework in commit fbcc02561359 ("xfs: Introduce
> writeback context for writepages") introduced a subtle change in
> behavior with regard to the block mapping used across the
> ->writepages() sequence. The previous xfs_cluster_write() code would
> only flush pages up to EOF at the time of the writepage, thus
> ensuring that any pages due to file-extending writes would be
> handled on a separate cycle and with a new, updated block mapping.
>
> The updated code establishes a block mapping in xfs_writepage_map()
> that could extend beyond EOF if the file has post-eof preallocation.
> Because we now use the generic writeback infrastructure and pass the
> cached mapping to each writepage call, there is no implicit EOF
> limit in place. If eofblocks trimming occurs during ->writepages(),
> any post-eof portion of the cached mapping becomes invalid. The
> eofblocks code has no means to serialize against writeback because
> there are no pages associated with post-eof blocks. Therefore if an
> eofblocks trim occurs and is followed by a file-extending buffered
> write, not only has the mapping become invalid, but we could end up
> writing a page to disk based on the invalid mapping.
>
> Consider the following sequence of events:
>
> - A buffered write creates a delalloc extent and post-eof
> speculative preallocation.
> - Writeback starts and on the first writepage cycle, the delalloc
> extent is converted to real blocks (including the post-eof blocks)
> and the mapping is cached.
> - The file is closed and xfs_release() trims post-eof blocks. The
> cached writeback mapping is now invalid.
> - Another buffered write appends the file with a delalloc extent.
> - The concurrent writeback cycle picks up the just written page
> because the writeback range end is LLONG_MAX. xfs_writepage_map()
> attributes it to the (now invalid) cached mapping and writes the
> data to an incorrect location on disk (and where the file offset is
> still backed by a delalloc extent).
>
> This problem is reproduced by xfstests test generic/463, which
> triggers racing writes, appends, open/closes and writeback requests.
>
> To address this problem, trim the mapping used during writeback to
> within EOF when the mapping is created. This ensures the mapping is
> revalidated for any pages encountered beyond EOF as of the time the
> current mapping was cached.
>
> Reported-by: Eryu Guan <eguan@redhat.com>
> Diagnosed-by: Eryu Guan <eguan@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>
> Hi all,
>
> This is a followup to the issue Eryu tracked down, described here[1].
>
> Note that this patch will not deal with any writeback mapping validity
> issues not associated with eofblocks management. Dave is working on a
> more generic approach to deal with such problems. This patch is intended
> to be a targeted and backportable fix for the regression in the
> writeback code.
>
> Brian
>
> [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
>
> fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
> fs/xfs/libxfs/xfs_bmap.h | 1 +
> fs/xfs/xfs_aops.c | 6 ++++--
> 3 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 044a363..dd3fb7b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3852,6 +3852,17 @@ xfs_trim_extent(
> }
> }
>
> +/* trim extent to within eof */
> +void
> +xfs_trim_extent_eof(
> + struct xfs_bmbt_irec *irec,
> + struct xfs_inode *ip)
> +
> +{
> + xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> + i_size_read(VFS_I(ip))));
> +}
Ok, so it's an unlocked, instantaneous sample of the inode size.
Truncate can race with this and still occur any time after we've
trimmed the extent but still have it cached.
As such, I'm thinking this EOF trimming it should be put in
xfs_imap_valid() - not xfs_map_blocks() - so it gets revalidated
every time we check to see if the map covers the current file
extent...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] xfs: trim writepage mapping to within eof
2017-10-12 21:22 ` Dave Chinner
@ 2017-10-13 11:42 ` Brian Foster
0 siblings, 0 replies; 6+ messages in thread
From: Brian Foster @ 2017-10-13 11:42 UTC (permalink / raw)
To: Dave Chinner; +Cc: linux-xfs
On Fri, Oct 13, 2017 at 08:22:44AM +1100, Dave Chinner wrote:
> On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> > The writeback rework in commit fbcc02561359 ("xfs: Introduce
> > writeback context for writepages") introduced a subtle change in
> > behavior with regard to the block mapping used across the
> > ->writepages() sequence. The previous xfs_cluster_write() code would
> > only flush pages up to EOF at the time of the writepage, thus
> > ensuring that any pages due to file-extending writes would be
> > handled on a separate cycle and with a new, updated block mapping.
> >
> > The updated code establishes a block mapping in xfs_writepage_map()
> > that could extend beyond EOF if the file has post-eof preallocation.
> > Because we now use the generic writeback infrastructure and pass the
> > cached mapping to each writepage call, there is no implicit EOF
> > limit in place. If eofblocks trimming occurs during ->writepages(),
> > any post-eof portion of the cached mapping becomes invalid. The
> > eofblocks code has no means to serialize against writeback because
> > there are no pages associated with post-eof blocks. Therefore if an
> > eofblocks trim occurs and is followed by a file-extending buffered
> > write, not only has the mapping become invalid, but we could end up
> > writing a page to disk based on the invalid mapping.
> >
> > Consider the following sequence of events:
> >
> > - A buffered write creates a delalloc extent and post-eof
> > speculative preallocation.
> > - Writeback starts and on the first writepage cycle, the delalloc
> > extent is converted to real blocks (including the post-eof blocks)
> > and the mapping is cached.
> > - The file is closed and xfs_release() trims post-eof blocks. The
> > cached writeback mapping is now invalid.
> > - Another buffered write appends the file with a delalloc extent.
> > - The concurrent writeback cycle picks up the just written page
> > because the writeback range end is LLONG_MAX. xfs_writepage_map()
> > attributes it to the (now invalid) cached mapping and writes the
> > data to an incorrect location on disk (and where the file offset is
> > still backed by a delalloc extent).
> >
> > This problem is reproduced by xfstests test generic/463, which
> > triggers racing writes, appends, open/closes and writeback requests.
> >
> > To address this problem, trim the mapping used during writeback to
> > within EOF when the mapping is created. This ensures the mapping is
> > revalidated for any pages encountered beyond EOF as of the time the
> > current mapping was cached.
> >
> > Reported-by: Eryu Guan <eguan@redhat.com>
> > Diagnosed-by: Eryu Guan <eguan@redhat.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >
> > Hi all,
> >
> > This is a followup to the issue Eryu tracked down, described here[1].
> >
> > Note that this patch will not deal with any writeback mapping validity
> > issues not associated with eofblocks management. Dave is working on a
> > more generic approach to deal with such problems. This patch is intended
> > to be a targeted and backportable fix for the regression in the
> > writeback code.
> >
> > Brian
> >
> > [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
> >
> > fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
> > fs/xfs/libxfs/xfs_bmap.h | 1 +
> > fs/xfs/xfs_aops.c | 6 ++++--
> > 3 files changed, 16 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 044a363..dd3fb7b 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -3852,6 +3852,17 @@ xfs_trim_extent(
> > }
> > }
> >
> > +/* trim extent to within eof */
> > +void
> > +xfs_trim_extent_eof(
> > + struct xfs_bmbt_irec *irec,
> > + struct xfs_inode *ip)
> > +
> > +{
> > + xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> > + i_size_read(VFS_I(ip))));
> > +}
>
> Ok, so it's an unlocked, instantaneous sample of the inode size.
> Truncate can race with this and still occur any time after we've
> trimmed the extent but still have it cached.
>
Yeah, but note that this is really only intended to deal with writeback
racing with eofblocks trimming. I'm not sure we can fully close any
other truncate/writeback issues without your broader, more generic
mapping invalidation work.
With regard to eofblocks trimming, I don't think it can hurt us once
we've trimmed the cached mapping once. Any new eofblocks that come in
due to new buffered writes aren't discovered until we acquire a new
mapping. Truncates up or down before we actually trim the cached mapping
basically also rule out eofblocks trims on file release causing a
problem for the writeback cycle.
> As such, I'm thinking this EOF trimming it should be put in
> xfs_imap_valid() - not xfs_map_blocks() - so it gets revalidated
> every time we check to see if the map covers the current file
> extent...
>
I'm not sure it's necessary, but it seems Ok to me to be slightly more
aggressive in the validation as long as it's clear (which I think it is
;P) that it isn't intended to technically close any issues unrelated to
eofblocks. I'll give it a shot.
Brian
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
> --
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^ permalink raw reply [flat|nested] 6+ messages in thread