btrfs: add LOGICAL_INO_V2 flag for avoiding the tree modification log

At Meta, we use the LOGICAL_INO_V2 ioctl to estimate sharing-aware space
usage. We sample the logical address space and resolve each sample back
to the inodes which reference it. The measurement is statistical, so
with enough samples it gives good bounds on real usage.

Currently in the LOGICAL_INO_V2 ioctl path, iterate_extent_inodes()
(which can walk backrefs either against the current trees or against the
commit roots) acquires a tree modification log sequence number when
walking the current trees, since those trees are being modified while
the walk is in progress. The log is then enabled filesystem wide for as
long as any such walk is running, and is only disabled again once the
last walk has finished.

While it is enabled the cost impacts the whole filesystem, not only the
caller. Modifications of interior nodes of the extent tree and of
subvolume trees have to be recorded, serialized on a single filesystem
wide lock. Delayed refs with a sequence number at or above the oldest
active one can not be run (the run path bails out with -EAGAIN in
btrfs_run_delayed_refs_for_head()), and metadata refs above it can not
be merged, so they accumulate and that work is deferred to transaction
commit.

Currently, userspace has no way to ask for anything cheaper. Whenever a
transaction is running, the walk uses the tree modification log, even if
the caller would tolerate results that could be up to one commit interval
stale.

Give userspace this option by adding BTRFS_LOGICAL_INO_ARGS_COMMIT_ROOT,
which makes the backref walk use the commit roots and therefore never
enables the tree modification log. Behavior is unchanged unless the flag
gets explicitly passed by userspace. iterate_extent_inodes() already
takes a search_commit_root argument for this, which scrub, send and the
data relocation warning path all pass as true. The flag simply plumbs it
through to userspace.

Assisted-by: LLM
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index 0a13baf3d8d1..b6f196bfa600 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -732,6 +732,16 @@ struct btrfs_ioctl_logical_ino_args {
  */
 #define BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET	(1ULL << 0)
 
+/*
+ * Resolve backrefs against the commit roots instead of the current trees.
+ *
+ * Resolving against the current trees is more expensive for the filesystem as
+ * a whole, not only for the caller. Callers that do not need to observe the
+ * currently running transaction should set this. The result can be up to one
+ * commit interval stale.
+ */
+#define BTRFS_LOGICAL_INO_ARGS_COMMIT_ROOT	(1ULL << 1)
+
 enum btrfs_dev_stat_values {
 	/* disk I/O failure stats */
 	BTRFS_DEV_STAT_WRITE_ERRS, /* EIO or EREMOTEIO from lower layers */