[PATCH 0/2] btrfs: some speedup with fiemap when leaves are shared

[PATCH 0/2] btrfs: some speedup with fiemap when leaves are shared

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Add a couple assertions for the fiemap code that checks if extents are
shared and speedup the extent sharedness check when we already know the
current leaf is shared. More details on the changelogs.

Filipe Manana (2):
  btrfs: assert commit root semaphore is held when accessing backref cache
  btrfs: skip backref walking during fiemap if we know the leaf is shared

 fs/btrfs/backref.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

-- 
2.35.1

[PATCH 1/2] btrfs: assert commit root semaphore is held when accessing backref cache

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

During fiemap, when accessing the cache that stores the sharedness of an
extent, we need to either be holding a transaction handle or the commit
root semaphore. I left comments about this in the comment that precedes
store_backref_shared_cache() and lookup_backref_shared_cache(), but have
actually not enforced it through assertions. So assert that the commit
root semaphore is held if we are not holding a transaction handle.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/backref.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 46851511b661..f846fec08c86 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1252,8 +1252,12 @@ static bool lookup_backref_shared_cache(struct btrfs_backref_share_check_ctx *ct
 					struct btrfs_root *root,
 					u64 bytenr, int level, bool *is_shared)
 {
+	const struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_backref_shared_cache_entry *entry;
 
+	if (!current->journal_info)
+		lockdep_assert_held(&fs_info->commit_root_sem);
+
 	if (!ctx->use_path_cache)
 		return false;
 
@@ -1288,7 +1292,7 @@ static bool lookup_backref_shared_cache(struct btrfs_backref_share_check_ctx *ct
 	 * could be a snapshot sharing this extent buffer.
 	 */
 	if (entry->is_shared &&
-	    entry->gen != btrfs_get_last_root_drop_gen(root->fs_info))
+	    entry->gen != btrfs_get_last_root_drop_gen(fs_info))
 		return false;
 
 	*is_shared = entry->is_shared;
@@ -1318,9 +1322,13 @@ static void store_backref_shared_cache(struct btrfs_backref_share_check_ctx *ctx
 				       struct btrfs_root *root,
 				       u64 bytenr, int level, bool is_shared)
 {
+	const struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_backref_shared_cache_entry *entry;
 	u64 gen;
 
+	if (!current->journal_info)
+		lockdep_assert_held(&fs_info->commit_root_sem);
+
 	if (!ctx->use_path_cache)
 		return;
 
@@ -1336,7 +1344,7 @@ static void store_backref_shared_cache(struct btrfs_backref_share_check_ctx *ctx
 	ASSERT(level >= 0);
 
 	if (is_shared)
-		gen = btrfs_get_last_root_drop_gen(root->fs_info);
+		gen = btrfs_get_last_root_drop_gen(fs_info);
 	else
 		gen = btrfs_root_last_snapshot(&root->root_item);
 
-- 
2.35.1

[PATCH 2/2] btrfs: skip backref walking during fiemap if we know the leaf is shared

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

During fiemap, when checking if a data extent is shared we are doing the
backref walking even if we already know the leaf is shared, which is a
waste of time since if the leaf shared then the data extent is also
shared. So skip the backref walking when we know we are in a shared leaf.

The following test was measures the gains for a case where all leaves
are shared due to a snapshot:

   $ cat test.sh
   #!/bin/bash

   DEV=/dev/sdj
   MNT=/mnt/sdj

   umount $DEV &> /dev/null
   mkfs.btrfs -f $DEV
   # Use compression to quickly create files with a lot of extents
   # (each with a size of 128K).
   mount -o compress=lzo $DEV $MNT

   # 40G gives 327680 extents, each with a size of 128K.
   xfs_io -f -c "pwrite -S 0xab -b 1M 0 40G" $MNT/foobar

   # Add some more files to increase the size of the fs and extent
   # trees (in the real world there's a lot of files and extents
   # from other files).
   xfs_io -f -c "pwrite -S 0xcd -b 1M 0 20G" $MNT/file1
   xfs_io -f -c "pwrite -S 0xef -b 1M 0 20G" $MNT/file2
   xfs_io -f -c "pwrite -S 0x73 -b 1M 0 20G" $MNT/file3

   # Create a snapshot so all the extents become indirectly shared
   # through subtrees, with a generation less than or equals to the
   # generation used to create the snapshot.
   btrfs subvolume snapshot -r $MNT $MNT/snap1

   # Unmount and mount again to clear cached metadata.
   umount $MNT
   mount -o compress=lzo $DEV $MNT

   start=$(date +%s%N)
   # The filefrag tool  uses the fiemap ioctl.
   filefrag $MNT/foobar
   end=$(date +%s%N)
   dur=$(( (end - start) / 1000000 ))
   echo "fiemap took $dur milliseconds (metadata not cached)"
   echo

   start=$(date +%s%N)
   filefrag $MNT/foobar
   end=$(date +%s%N)
   dur=$(( (end - start) / 1000000 ))
   echo "fiemap took $dur milliseconds (metadata cached)"

   umount $MNT

The results were the following on a non-debug kernel (Debian's default
kernel config).

Before this patch:

   (...)
   /mnt/sdi/foobar: 327680 extents found
   fiemap took 1821 milliseconds (metadata not cached)

   /mnt/sdi/foobar: 327680 extents found
   fiemap took 399 milliseconds (metadata cached)

After this patch:

   (...)
   /mnt/sdi/foobar: 327680 extents found
   fiemap took 591 milliseconds (metadata not cached)

   /mnt/sdi/foobar: 327680 extents found
   fiemap took 123 milliseconds (metadata cached)

That's a speedup of 3.1x and 3.2x.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/backref.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index f846fec08c86..90e40d5ceccd 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1872,6 +1872,8 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr,
 		.have_delayed_delete_refs = false,
 	};
 	int level;
+	bool leaf_cached;
+	bool leaf_is_shared;
 
 	for (int i = 0; i < BTRFS_BACKREF_CTX_PREV_EXTENTS_SIZE; i++) {
 		if (ctx->prev_extents_cache[i].bytenr == bytenr)
@@ -1893,6 +1895,23 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr,
 		walk_ctx.time_seq = elem.seq;
 	}
 
+	ctx->use_path_cache = true;
+
+	/*
+	 * We may have previously determined that the current leaf is shared.
+	 * If it is, then we have a data extent that is shared due to a shared
+	 * subtree (caused by snapshotting) and we don't need to check for data
+	 * backrefs. If the leaf is not shared, then we must do backref walking
+	 * to determine if the data extent is shared through reflinks.
+	 */
+	leaf_cached = lookup_backref_shared_cache(ctx, root,
+						  ctx->curr_leaf_bytenr, 0,
+						  &leaf_is_shared);
+	if (leaf_cached && leaf_is_shared) {
+		ret = 1;
+		goto out_trans;
+	}
+
 	walk_ctx.ignore_extent_item_pos = true;
 	walk_ctx.trans = trans;
 	walk_ctx.fs_info = fs_info;
@@ -1901,7 +1920,6 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr,
 	/* -1 means we are in the bytenr of the data extent. */
 	level = -1;
 	ULIST_ITER_INIT(&uiter);
-	ctx->use_path_cache = true;
 	while (1) {
 		bool is_shared;
 		bool cached;
@@ -1972,6 +1990,7 @@ int btrfs_is_data_extent_shared(struct btrfs_inode *inode, u64 bytenr,
 		ctx->prev_extents_cache_slot = slot;
 	}
 
+out_trans:
 	if (trans) {
 		btrfs_put_tree_mod_seq(fs_info, &elem);
 		btrfs_end_transaction(trans);
-- 
2.35.1

Re: [PATCH 1/2] btrfs: assert commit root semaphore is held when accessing backref cache

[Johannes Thumshirn]

Looks good,
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Re: [PATCH 0/2] btrfs: some speedup with fiemap when leaves are shared

[David Sterba]

On Tue, Jan 17, 2023 at 11:21:37AM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Add a couple assertions for the fiemap code that checks if extents are
> shared and speedup the extent sharedness check when we already know the
> current leaf is shared. More details on the changelogs.
> 
> Filipe Manana (2):
>   btrfs: assert commit root semaphore is held when accessing backref cache
>   btrfs: skip backref walking during fiemap if we know the leaf is shared

Added to misc-next, thanks.