[PATCH 0/2] btrfs: a couple fixes for extent map merging and defrag

[PATCH 0/2] btrfs: a couple fixes for extent map merging and defrag

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Some fixes for extent map merging not happened when it should (which avoids
saving some memory) and defrag not merging contiguous extents when it should
due to merged extent maps. Details in the change logs.

Filipe Manana (2):
  btrfs: fix extent map merging not happening for adjacent extents
  btrfs: fix defrag not merging contiguous extents due to merged extent maps

 fs/btrfs/defrag.c     | 10 +++++-----
 fs/btrfs/extent_map.c |  7 ++++++-
 2 files changed, 11 insertions(+), 6 deletions(-)

-- 
2.45.2

[PATCH 1/2] btrfs: fix extent map merging not happening for adjacent extents

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

If we have 3 or more adjacent extents in a file, that is, consecutive file
extent items pointing to adjacent extents, within a contiguous file range
and compatible flags, we end up not merging all the extents into a single
extent map.

For example:

  $ mkfs.btrfs -f /dev/sdc
  $ mount /dev/sdc /mnt/sdc

  $ xfs_io -f -d -c "pwrite -b 64K 0 64K" \
                 -c "pwrite -b 64K 64K 64K" \
                 -c "pwrite -b 64K 128K 64K" \
                 -c "pwrite -b 64K 192K 64K" \
                 /mnt/sdc/foo

After all the ordered extents complete we unpin the extent maps and try
to merge them, but instead of getting a single extent map we get two
because:

1) When the first ordered extent completes (file range [0, 64K)) we
   unpin its extent map and attempt to merge it with the extent map for
   the range [64K, 128K), but we can't because that extent map is still
   pinned;

2) When the second ordered extent completes (file range [64K, 128K)), we
   unpin its extent map and merge it with the previous extent map, for
   file range [0, 64K), but we can't merge with the next extent map, for
   the file range [128K, 192K), because this one is still pinned.

   The merged extent map for the file range [0, 128K) gets the flag
   EXTENT_MAP_MERGED set;

3) When the third ordered extent completes (file range [128K, 192K)), we
   unpin its exent map and attempt to merge it with the previous extent
   map, for file range [0, 128K), but we can't because that extent map
   has the flag EXTENT_MAP_MERGED set (mergeable_maps() returns false
   due to different flags) while the extent map for the range [128K, 192K)
   doesn't have that flag set.

   We also can't merge it with the next extent map, for file range
   [192K, 256K), because that one is still pinned.

   At this moment we have 3 extent maps:

   One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
   One for file range [128K, 192K).
   One for file range [192K, 256K) which is still pinned;

4) When the fourth and final extent completes (file range [192K, 256K)),
   we unpin its extent map and attempt to merge it with the previous
   extent map, for file range [128K, 192K), which succeeds since none
   of these extent maps have the EXTENT_MAP_MERGED flag set.

   So we end up with 2 extent maps:

   One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
   One for file range [128K, 256K), with the flag EXTENT_MAP_MERGED set.

   Since after merging extent maps we don't attempt to merge again, that
   is, merge the resulting extent map with the one that is now preceding
   it (and the one following it), we end up with those two extent maps,
   when we could have had a single extent map to represent the whole file.

Fix this by making mergeable_maps() ignore the EXTENT_MAP_MERGED flag.
While this doesn't present any functional issue, it prevents the merging
of extent maps which allows to save memory, and can make defrag not
merging extents too (that will be addressed in the next patch).

Fixes: 199257a78bb0 ("btrfs: defrag: don't use merged extent map for their generation check")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/extent_map.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 1f85b54c8f0c..67ce85ff0ae2 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -233,7 +233,12 @@ static bool mergeable_maps(const struct extent_map *prev, const struct extent_ma
 	if (extent_map_end(prev) != next->start)
 		return false;
 
-	if (prev->flags != next->flags)
+	/*
+	 * The merged flag is not an on-disk flag, it just indicates we had the
+	 * extent maps of 2 (or more) adjacent extents merged, so factor it out.
+	 */
+	if ((prev->flags & ~EXTENT_FLAG_MERGED) !=
+	    (next->flags & ~EXTENT_FLAG_MERGED))
 		return false;
 
 	if (next->disk_bytenr < EXTENT_MAP_LAST_BYTE - 1)
-- 
2.45.2

Re: [PATCH 1/2] btrfs: fix extent map merging not happening for adjacent extents

[Anand Jain]

On 30/10/24 01:22, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If we have 3 or more adjacent extents in a file, that is, consecutive file
> extent items pointing to adjacent extents, within a contiguous file range
> and compatible flags, we end up not merging all the extents into a single
> extent map.
> 
> For example:
> 
>    $ mkfs.btrfs -f /dev/sdc
>    $ mount /dev/sdc /mnt/sdc
> 
>    $ xfs_io -f -d -c "pwrite -b 64K 0 64K" \
>                   -c "pwrite -b 64K 64K 64K" \
>                   -c "pwrite -b 64K 128K 64K" \
>                   -c "pwrite -b 64K 192K 64K" \
>                   /mnt/sdc/foo
> 
> After all the ordered extents complete we unpin the extent maps and try
> to merge them, but instead of getting a single extent map we get two
> because:
> 
> 1) When the first ordered extent completes (file range [0, 64K)) we
>     unpin its extent map and attempt to merge it with the extent map for
>     the range [64K, 128K), but we can't because that extent map is still
>     pinned;
> 
> 2) When the second ordered extent completes (file range [64K, 128K)), we
>     unpin its extent map and merge it with the previous extent map, for
>     file range [0, 64K), but we can't merge with the next extent map, for
>     the file range [128K, 192K), because this one is still pinned.
> 
>     The merged extent map for the file range [0, 128K) gets the flag
>     EXTENT_MAP_MERGED set;
> 
> 3) When the third ordered extent completes (file range [128K, 192K)), we
>     unpin its exent map and attempt to merge it with the previous extent
>     map, for file range [0, 128K), but we can't because that extent map
>     has the flag EXTENT_MAP_MERGED set (mergeable_maps() returns false
>     due to different flags) while the extent map for the range [128K, 192K)
>     doesn't have that flag set.
> 
>     We also can't merge it with the next extent map, for file range
>     [192K, 256K), because that one is still pinned.
> 
>     At this moment we have 3 extent maps:
> 
>     One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
>     One for file range [128K, 192K).
>     One for file range [192K, 256K) which is still pinned;
> 
> 4) When the fourth and final extent completes (file range [192K, 256K)),
>     we unpin its extent map and attempt to merge it with the previous
>     extent map, for file range [128K, 192K), which succeeds since none
>     of these extent maps have the EXTENT_MAP_MERGED flag set.
> 
>     So we end up with 2 extent maps:
> 
>     One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
>     One for file range [128K, 256K), with the flag EXTENT_MAP_MERGED set.
> 
>     Since after merging extent maps we don't attempt to merge again, that
>     is, merge the resulting extent map with the one that is now preceding
>     it (and the one following it), we end up with those two extent maps,
>     when we could have had a single extent map to represent the whole file.
> 
> Fix this by making mergeable_maps() ignore the EXTENT_MAP_MERGED flag.
> While this doesn't present any functional issue, it prevents the merging
> of extent maps which allows to save memory, and can make defrag not
> merging extents too (that will be addressed in the next patch).
> 

Why don’t the extents merge, even after mount-recycles and multiple 
manual defrag runs, without this fix?

Thanks, Anand


> Fixes: 199257a78bb0 ("btrfs: defrag: don't use merged extent map for their generation check")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   fs/btrfs/extent_map.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
> index 1f85b54c8f0c..67ce85ff0ae2 100644
> --- a/fs/btrfs/extent_map.c
> +++ b/fs/btrfs/extent_map.c
> @@ -233,7 +233,12 @@ static bool mergeable_maps(const struct extent_map *prev, const struct extent_ma
>   	if (extent_map_end(prev) != next->start)
>   		return false;
>   
> -	if (prev->flags != next->flags)
> +	/*
> +	 * The merged flag is not an on-disk flag, it just indicates we had the
> +	 * extent maps of 2 (or more) adjacent extents merged, so factor it out.
> +	 */
> +	if ((prev->flags & ~EXTENT_FLAG_MERGED) !=
> +	    (next->flags & ~EXTENT_FLAG_MERGED))
>   		return false;
>   
>   	if (next->disk_bytenr < EXTENT_MAP_LAST_BYTE - 1)

Re: [PATCH 1/2] btrfs: fix extent map merging not happening for adjacent extents

[Filipe Manana]

On Wed, Oct 30, 2024 at 12:48 AM Anand Jain <anand.jain@oracle.com> wrote:
>
> On 30/10/24 01:22, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > If we have 3 or more adjacent extents in a file, that is, consecutive file
> > extent items pointing to adjacent extents, within a contiguous file range
> > and compatible flags, we end up not merging all the extents into a single
> > extent map.
> >
> > For example:
> >
> >    $ mkfs.btrfs -f /dev/sdc
> >    $ mount /dev/sdc /mnt/sdc
> >
> >    $ xfs_io -f -d -c "pwrite -b 64K 0 64K" \
> >                   -c "pwrite -b 64K 64K 64K" \
> >                   -c "pwrite -b 64K 128K 64K" \
> >                   -c "pwrite -b 64K 192K 64K" \
> >                   /mnt/sdc/foo
> >
> > After all the ordered extents complete we unpin the extent maps and try
> > to merge them, but instead of getting a single extent map we get two
> > because:
> >
> > 1) When the first ordered extent completes (file range [0, 64K)) we
> >     unpin its extent map and attempt to merge it with the extent map for
> >     the range [64K, 128K), but we can't because that extent map is still
> >     pinned;
> >
> > 2) When the second ordered extent completes (file range [64K, 128K)), we
> >     unpin its extent map and merge it with the previous extent map, for
> >     file range [0, 64K), but we can't merge with the next extent map, for
> >     the file range [128K, 192K), because this one is still pinned.
> >
> >     The merged extent map for the file range [0, 128K) gets the flag
> >     EXTENT_MAP_MERGED set;
> >
> > 3) When the third ordered extent completes (file range [128K, 192K)), we
> >     unpin its exent map and attempt to merge it with the previous extent
> >     map, for file range [0, 128K), but we can't because that extent map
> >     has the flag EXTENT_MAP_MERGED set (mergeable_maps() returns false
> >     due to different flags) while the extent map for the range [128K, 192K)
> >     doesn't have that flag set.
> >
> >     We also can't merge it with the next extent map, for file range
> >     [192K, 256K), because that one is still pinned.
> >
> >     At this moment we have 3 extent maps:
> >
> >     One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
> >     One for file range [128K, 192K).
> >     One for file range [192K, 256K) which is still pinned;
> >
> > 4) When the fourth and final extent completes (file range [192K, 256K)),
> >     we unpin its extent map and attempt to merge it with the previous
> >     extent map, for file range [128K, 192K), which succeeds since none
> >     of these extent maps have the EXTENT_MAP_MERGED flag set.
> >
> >     So we end up with 2 extent maps:
> >
> >     One for file range [0, 128K), with the flag EXTENT_MAP_MERGED set.
> >     One for file range [128K, 256K), with the flag EXTENT_MAP_MERGED set.
> >
> >     Since after merging extent maps we don't attempt to merge again, that
> >     is, merge the resulting extent map with the one that is now preceding
> >     it (and the one following it), we end up with those two extent maps,
> >     when we could have had a single extent map to represent the whole file.
> >
> > Fix this by making mergeable_maps() ignore the EXTENT_MAP_MERGED flag.
> > While this doesn't present any functional issue, it prevents the merging
> > of extent maps which allows to save memory, and can make defrag not
> > merging extents too (that will be addressed in the next patch).
> >
>
> Why don’t the extents merge, even after mount-recycles and multiple
> manual defrag runs, without this fix?

Why do you think mount recycles would make any difference? Whenever
extent maps are merged they get the merge flag set.
As for defrag and merged extent maps, that's explained in the next patch.

>
> Thanks, Anand
>
>
> > Fixes: 199257a78bb0 ("btrfs: defrag: don't use merged extent map for their generation check")
> > Signed-off-by: Filipe Manana <fdmanana@suse.com>
> > ---
> >   fs/btrfs/extent_map.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
> > index 1f85b54c8f0c..67ce85ff0ae2 100644
> > --- a/fs/btrfs/extent_map.c
> > +++ b/fs/btrfs/extent_map.c
> > @@ -233,7 +233,12 @@ static bool mergeable_maps(const struct extent_map *prev, const struct extent_ma
> >       if (extent_map_end(prev) != next->start)
> >               return false;
> >
> > -     if (prev->flags != next->flags)
> > +     /*
> > +      * The merged flag is not an on-disk flag, it just indicates we had the
> > +      * extent maps of 2 (or more) adjacent extents merged, so factor it out.
> > +      */
> > +     if ((prev->flags & ~EXTENT_FLAG_MERGED) !=
> > +         (next->flags & ~EXTENT_FLAG_MERGED))
> >               return false;
> >
> >       if (next->disk_bytenr < EXTENT_MAP_LAST_BYTE - 1)
>

[PATCH 2/2] btrfs: fix defrag not merging contiguous extents due to merged extent maps

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

When running defrag (manual defrag) against a file that has extents that
are contiguous and we already have the respective extent maps loaded and
merged, we end up not defragging the range covered by those contiguous
extents. This happens when we have an extent map that was the result of
merging multiple extent maps for contiguous extents and the length of the
merged extent map is greater than or equals to the defrag threshold
length.

The script below reproduces this scenario:

   $ cat test.sh
   #!/bin/bash

   DEV=/dev/sdi
   MNT=/mnt/sdi

   mkfs.btrfs -f $DEV
   mount $DEV $MNT

   # Create a 256K file with 4 extents of 64K each.
   xfs_io -f -c "falloc 0 64K" \
             -c "pwrite 0 64K" \
             -c "falloc 64K 64K" \
             -c "pwrite 64K 64K" \
             -c "falloc 128K 64K" \
             -c "pwrite 128K 64K" \
             -c "falloc 192K 64K" \
             -c "pwrite 192K 64K" \
             $MNT/foo

   umount $MNT
   echo -n "Initial number of file extent items: "
   btrfs inspect-internal dump-tree -t 5 $DEV | grep EXTENT_DATA | wc -l

   mount $DEV $MNT
   # Read the whole file in order to load and merge extent maps.
   cat $MNT/foo > /dev/null

   btrfs filesystem defragment -t 128K $MNT/foo
   umount $MNT
   echo -n "Number of file extent items after defrag with 128K threshold: "
   btrfs inspect-internal dump-tree -t 5 $DEV | grep EXTENT_DATA | wc -l

   mount $DEV $MNT
   # Read the whole file in order to load and merge extent maps.
   cat $MNT/foo > /dev/null

   btrfs filesystem defragment -t 256K $MNT/foo
   umount $MNT
   echo -n "Number of file extent items after defrag with 256K threshold: "
   btrfs inspect-internal dump-tree -t 5 $DEV | grep EXTENT_DATA | wc -l

Running it:

   $ ./test.sh
   Initial number of file extent items: 4
   Number of file extent items after defrag with 128K threshold: 4
   Number of file extent items after defrag with 256K threshold: 4

The 4 extents don't get merged because we have an extent map with a size
of 256K that is the result of merging the indiviual extent maps for each
of the four 64K extents and at defrag_lookup_extent() we have a value of
zero for the generation threshold ('newer_than' argument) since this is a
manual defrag. As a consequence we don't call defrag_get_extent() to get
an extent map representing a single file extent item in the inode's
subvolume tree, so we end up using the merged extent map at
defrag_collect_targets() and decide not to defrag.

Fix this by updating defrag_lookup_extent() to always discard extent maps
that were merged and call defrag_get_extent() regardless of the minimum
generation threshold ('newer_than' argument).

A test case for fstests will be sent along soon.

Fixes: 199257a78bb0 ("btrfs: defrag: don't use merged extent map for their generation check")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/defrag.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/defrag.c b/fs/btrfs/defrag.c
index b95ef44c326b..968dae953948 100644
--- a/fs/btrfs/defrag.c
+++ b/fs/btrfs/defrag.c
@@ -763,12 +763,12 @@ static struct extent_map *defrag_lookup_extent(struct inode *inode, u64 start,
 	 * We can get a merged extent, in that case, we need to re-search
 	 * tree to get the original em for defrag.
 	 *
-	 * If @newer_than is 0 or em::generation < newer_than, we can trust
-	 * this em, as either we don't care about the generation, or the
-	 * merged extent map will be rejected anyway.
+	 * This is because even if we have adjacent extents that are contiguous
+	 * and compatible (same type and flags), we still want to defrag them
+	 * so that we use less metadata (extent items in the extent tree and
+	 * file extent items in the inode's subvolume tree).
 	 */
-	if (em && (em->flags & EXTENT_FLAG_MERGED) &&
-	    newer_than && em->generation >= newer_than) {
+	if (em && (em->flags & EXTENT_FLAG_MERGED)) {
 		free_extent_map(em);
 		em = NULL;
 	}
-- 
2.45.2

Re: [PATCH 0/2] btrfs: a couple fixes for extent map merging and defrag

[Qu Wenruo]

在 2024/10/30 03:52, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
>
> Some fixes for extent map merging not happened when it should (which avoids
> saving some memory) and defrag not merging contiguous extents when it should
> due to merged extent maps. Details in the change logs.
>
> Filipe Manana (2):
>    btrfs: fix extent map merging not happening for adjacent extents
>    btrfs: fix defrag not merging contiguous extents due to merged extent maps

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

>
>   fs/btrfs/defrag.c     | 10 +++++-----
>   fs/btrfs/extent_map.c |  7 ++++++-
>   2 files changed, 11 insertions(+), 6 deletions(-)
>