Re: [PATCH v1] btrfs: add LOGICAL_INO_V2 flag for avoiding the tree modification log

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Tue, Sep 22, 2026 at 01:12:57PM -0700, Joanne Koong wrote:
> 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.

I was about to send the same patch after tracking down some performance
issues.  This is the same use case as btdu, and bees goes a lot faster
without the tree mod log as well.  Much appreciated!

> Assisted-by: LLM
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  fs/btrfs/backref.c         |  6 ++++--
>  fs/btrfs/backref.h         |  3 ++-
>  fs/btrfs/ioctl.c           |  9 +++++++--
>  include/uapi/linux/btrfs.h | 10 ++++++++++
>  4 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index 1be632c742bd..364ec3ad7d8a 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -2549,7 +2549,8 @@ static int build_ino_list(u64 inum, u64 offset, u64 num_bytes, u64 root, void *c
>  }
>  
>  int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
> -				void *ctx, bool ignore_offset)
> +				void *ctx, bool ignore_offset,
> +				bool search_commit_root)
>  {
>  	struct btrfs_backref_walk_ctx walk_ctx = { 0 };
>  	int ret;
> @@ -2575,7 +2576,8 @@ int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
>  		walk_ctx.extent_item_pos = logical - found_key.objectid;
>  	walk_ctx.fs_info = fs_info;
>  
> -	return iterate_extent_inodes(&walk_ctx, false, build_ino_list, ctx);
> +	return iterate_extent_inodes(&walk_ctx, search_commit_root,
> +				     build_ino_list, ctx);
>  }
>  
>  static int inode_to_path(u64 inum, u32 name_len, unsigned long name_off,
> diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
> index 179791de6b19..f8dc8a3723cf 100644
> --- a/fs/btrfs/backref.h
> +++ b/fs/btrfs/backref.h
> @@ -226,7 +226,8 @@ int iterate_extent_inodes(struct btrfs_backref_walk_ctx *ctx,
>  			  iterate_extent_inodes_t *iterate, void *user_ctx);
>  
>  int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
> -				void *ctx, bool ignore_offset);
> +				void *ctx, bool ignore_offset,
> +				bool search_commit_root);
>  
>  int paths_from_inode(u64 inum, struct inode_fs_paths *ipath);
>  
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 52aab510aea0..63257f8fb897 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -3286,6 +3286,7 @@ static long btrfs_ioctl_logical_to_ino(struct btrfs_fs_info *fs_info,
>  	struct btrfs_ioctl_logical_ino_args AUTO_KFREE(loi);
>  	struct btrfs_data_container AUTO_KVFREE(inodes);
>  	bool ignore_offset;
> +	bool search_commit_root;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
> @@ -3296,6 +3297,7 @@ static long btrfs_ioctl_logical_to_ino(struct btrfs_fs_info *fs_info,
>  
>  	if (version == 1) {
>  		ignore_offset = false;
> +		search_commit_root = false;
>  		size = min_t(u32, loi->size, SZ_64K);
>  	} else {
>  		/* All reserved bits must be 0 for now */
> @@ -3303,10 +3305,12 @@ static long btrfs_ioctl_logical_to_ino(struct btrfs_fs_info *fs_info,
>  			return -EINVAL;
>  
>  		/* Only accept flags we have defined so far */
> -		if (loi->flags & ~(BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET))
> +		if (loi->flags & ~(BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET |
> +				   BTRFS_LOGICAL_INO_ARGS_COMMIT_ROOT))
>  			return -EINVAL;
>  
>  		ignore_offset = loi->flags & BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET;
> +		search_commit_root = loi->flags & BTRFS_LOGICAL_INO_ARGS_COMMIT_ROOT;
>  		size = min_t(u32, loi->size, SZ_16M);
>  	}
>  
> @@ -3314,7 +3318,8 @@ static long btrfs_ioctl_logical_to_ino(struct btrfs_fs_info *fs_info,
>  	if (IS_ERR(inodes))
>  		return PTR_ERR(inodes);
>  
> -	ret = iterate_inodes_from_logical(loi->logical, fs_info, inodes, ignore_offset);
> +	ret = iterate_inodes_from_logical(loi->logical, fs_info, inodes,
> +					  ignore_offset, search_commit_root);
>  	if (ret == -EINVAL)
>  		return -ENOENT;
>  	if (ret < 0)
> 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 */
> -- 
> 2.52.0
> 
>