| Message ID | 3937cbcdf53440dcda98b64ce5e0e1cf8b15803d.1712929998.git.josef@toxicpanda.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] btrfs: check delayed refs when we're checking if a ref exists | expand |
On Fri, Apr 12, 2024 at 2:55 PM Josef Bacik <josef@toxicpanda.com> wrote: > > In the patch 78c52d9eb6b7 ("btrfs: check for refs on snapshot delete > resume") I added some code to handle file systems that had been > corrupted by a bug that incorrectly skipped updating the drop progress > key while dropping a snapshot. This code would check to see if we had > already deleted our reference for a child block, and skip the deletion > if we had already. > > Unfortunately there is a bug, as the check would only check the on-disk > references. I made an incorrect assumption that blocks in an already > deleted snapshot that was having the deletion resume on mount wouldn't > be modified. > > If we have 2 pending deleted snapshots that share blocks, we can easily > modify the rules for a block. Take the following example > > subvolume a exists, and subvolume b is a snapshot of subvolume a. They > share references to block 1. Block 1 will have 2 full references, one > for subvolume a and one for subvolume b, and it belongs to subvolume a > (btrfs_header_owner(block 1) == subvolume a). > > When deleting subvolume a, we will drop our full reference for block 1, > and because we are the owner we will drop our full reference for all of > block 1's children, convert block 1 to FULL BACKREF, and add a shared > reference to all of block 1's children. > > Then we will start the snapshot deletion of subvolume b. We look up the > extent info for block 1, which checks delayed refs and tells us that > FULL BACKREF is set, so sets parent to the bytenr of block 1. However > because this is a resumed snapshot deletion, we call into > check_ref_exists(). Because check_ref_exists() only looks at the disk, > it doesn't find the shared backref for the child of block 1, and thus > returns 0 and we skip deleting the reference for the child of block 1 > and continue. This orphans the child of block 1. > > The fix is to lookup the delayed refs, similar to what we do in > btrfs_lookup_extent_info(). However we only care about whether the > reference exists or not. If we fail to find our reference on disk, go > look up the bytenr in the delayed refs, and if it exists look for an > existing ref in the delayed ref head. If that exists then we know we > can delete the reference safely and carry on. If it doesn't exist we > know we have to skip over this block. > > This bug has existed since I introduced this fix, however requires > having multiple deleted snapshots pending when we unmount. We noticed > this in production because our shutdown path stops the container on the > system, which deletes a bunch of subvolumes, and then reboots the box. > This gives us plenty of opportunities to hit this issue. Looking at the > history we've seen this occasionally in production, but we had a big > spike recently thanks to faster machines getting jobs with multiple > subvolumes in the job. > > Chris Mason wrote a reproducer which does the following > > mount /dev/nvme4n1 /btrfs > btrfs subvol create /btrfs/s1 > simoop -E -f 4k -n 200000 -z /btrfs/s1 > while(true) ; do > btrfs subvol snap /btrfs/s1 /btrfs/s2 > simoop -f 4k -n 200000 -r 10 -z /btrfs/s2 > btrfs subvol snap /btrfs/s2 /btrfs/s3 > btrfs balance start -dusage=80 /btrfs > btrfs subvol del /btrfs/s2 /btrfs/s3 > umount /btrfs > btrfsck /dev/nvme4n1 || exit 1 > mount /dev/nvme4n1 /btrfs > done > > On the second loop this would fail consistently, with my patch it has > been running for hours and hasn't failed. > > I also used dm-log-writes to capture the state of the failure so I could > debug the problem. Using the existing failure case to test my patch > validated that it fixes the problem. > > Fixes: 78c52d9eb6b7 ("btrfs: check for refs on snapshot delete resume") > Signed-off-by: Josef Bacik <josef@toxicpanda.com> Reviewed-by: Filipe Manana <fdmanana@suse.com> Looks good, thanks. > --- > v1->v2: > - Fixed up style nits pointed out by Filipe. > - Fixed up the changelog to fix some wrong words and make things more clear. > > fs/btrfs/delayed-ref.c | 69 ++++++++++++++++++++++++++++++++++++++++++ > fs/btrfs/delayed-ref.h | 2 ++ > fs/btrfs/extent-tree.c | 51 +++++++++++++++++++++++++++---- > 3 files changed, 116 insertions(+), 6 deletions(-) > > diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c > index e44e62cf76bc..8c0970980acb 100644 > --- a/fs/btrfs/delayed-ref.c > +++ b/fs/btrfs/delayed-ref.c > @@ -1297,6 +1297,75 @@ btrfs_find_delayed_ref_head(struct btrfs_delayed_ref_root *delayed_refs, u64 byt > return find_ref_head(delayed_refs, bytenr, false); > } > > +static int find_comp(struct btrfs_delayed_ref_node *entry, u64 root, u64 parent) > +{ > + struct btrfs_delayed_tree_ref *ref; > + int type = parent ? BTRFS_SHARED_BLOCK_REF_KEY : BTRFS_TREE_BLOCK_REF_KEY; > + > + if (type < entry->type) > + return -1; > + if (type > entry->type) > + return 1; > + > + ref = btrfs_delayed_node_to_tree_ref(entry); > + if (type == BTRFS_TREE_BLOCK_REF_KEY) { > + if (root < ref->root) > + return -1; > + if (root > ref->root) > + return 1; > + } else { > + if (parent < ref->parent) > + return -1; > + if (parent > ref->parent) > + return 1; > + } > + return 0; > +} > + > +/* > + * Check to see if a given root/parent reference is attached to the head. This > + * only checks for BTRFS_ADD_DELAYED_REF references that match, as that > + * indicates the reference exists for the given root or parent. This is for > + * tree blocks only. > + * > + * @head: the head of the bytenr we're searching. > + * @root: the root objectid of the reference if it is a normal reference. > + * @parent: the parent if this is a shared backref. > + */ > +bool btrfs_find_delayed_tree_ref(struct btrfs_delayed_ref_head *head, > + u64 root, u64 parent) > +{ > + struct rb_node *node; > + bool found = false; > + > + lockdep_assert_held(&head->mutex); > + > + spin_lock(&head->lock); > + node = head->ref_tree.rb_root.rb_node; > + while (node) { > + struct btrfs_delayed_ref_node *entry; > + int ret; > + > + entry = rb_entry(node, struct btrfs_delayed_ref_node, ref_node); > + ret = find_comp(entry, root, parent); > + if (ret < 0) { > + node = node->rb_left; > + } else if (ret > 0) { > + node = node->rb_right; > + } else { > + /* > + * We only want to count ADD actions, as drops mean the > + * ref doesn't exist. > + */ > + if (entry->action == BTRFS_ADD_DELAYED_REF) > + found = true; > + break; > + } > + } > + spin_unlock(&head->lock); > + return found; > +} > + > void __cold btrfs_delayed_ref_exit(void) > { > kmem_cache_destroy(btrfs_delayed_ref_head_cachep); > diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h > index b291147cb8ab..34c34db01cc6 100644 > --- a/fs/btrfs/delayed-ref.h > +++ b/fs/btrfs/delayed-ref.h > @@ -397,6 +397,8 @@ int btrfs_delayed_refs_rsv_refill(struct btrfs_fs_info *fs_info, > void btrfs_migrate_to_delayed_refs_rsv(struct btrfs_fs_info *fs_info, > u64 num_bytes); > bool btrfs_check_space_for_delayed_refs(struct btrfs_fs_info *fs_info); > +bool btrfs_find_delayed_tree_ref(struct btrfs_delayed_ref_head *head, > + u64 root, u64 parent); > > /* > * helper functions to cast a node into its container > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > index 42314604906a..b21998c1e33f 100644 > --- a/fs/btrfs/extent-tree.c > +++ b/fs/btrfs/extent-tree.c > @@ -5428,23 +5428,62 @@ static int check_ref_exists(struct btrfs_trans_handle *trans, > struct btrfs_root *root, u64 bytenr, u64 parent, > int level) > { > + struct btrfs_delayed_ref_root *delayed_refs; > + struct btrfs_delayed_ref_head *head; > struct btrfs_path *path; > struct btrfs_extent_inline_ref *iref; > int ret; > + bool exists = false; > > path = btrfs_alloc_path(); > if (!path) > return -ENOMEM; > - > +again: > ret = lookup_extent_backref(trans, path, &iref, bytenr, > root->fs_info->nodesize, parent, > root->root_key.objectid, level, 0); > + if (ret != -ENOENT) { > + /* > + * If we get 0 then we found our reference, return 1, else > + * return the error if it's not -ENOENT; > + */ > + btrfs_free_path(path); > + return (ret < 0 ) ? ret : 1; > + } > + > + /* > + * We could have a delayed ref with this reference, so look it up while > + * we're holding the path open to make sure we don't race with the > + * delayed ref running. > + */ > + delayed_refs = &trans->transaction->delayed_refs; > + spin_lock(&delayed_refs->lock); > + head = btrfs_find_delayed_ref_head(delayed_refs, bytenr); > + if (!head) > + goto out; > + if (!mutex_trylock(&head->mutex)) { > + /* > + * We're contended, means that the delayed ref is running, get a > + * reference and wait for the ref head to be complete and then > + * try again. > + */ > + refcount_inc(&head->refs); > + spin_unlock(&delayed_refs->lock); > + > + btrfs_release_path(path); > + > + mutex_lock(&head->mutex); > + mutex_unlock(&head->mutex); > + btrfs_put_delayed_ref_head(head); > + goto again; > + } > + > + exists = btrfs_find_delayed_tree_ref(head, root->root_key.objectid, parent); > + mutex_unlock(&head->mutex); > +out: > + spin_unlock(&delayed_refs->lock); > btrfs_free_path(path); > - if (ret == -ENOENT) > - return 0; > - if (ret < 0) > - return ret; > - return 1; > + return exists ? 1 : 0; > } > > /* > -- > 2.43.0 > >
diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c index e44e62cf76bc..8c0970980acb 100644 --- a/fs/btrfs/delayed-ref.c +++ b/fs/btrfs/delayed-ref.c @@ -1297,6 +1297,75 @@ btrfs_find_delayed_ref_head(struct btrfs_delayed_ref_root *delayed_refs, u64 byt return find_ref_head(delayed_refs, bytenr, false); } +static int find_comp(struct btrfs_delayed_ref_node *entry, u64 root, u64 parent) +{ + struct btrfs_delayed_tree_ref *ref; + int type = parent ? BTRFS_SHARED_BLOCK_REF_KEY : BTRFS_TREE_BLOCK_REF_KEY; + + if (type < entry->type) + return -1; + if (type > entry->type) + return 1; + + ref = btrfs_delayed_node_to_tree_ref(entry); + if (type == BTRFS_TREE_BLOCK_REF_KEY) { + if (root < ref->root) + return -1; + if (root > ref->root) + return 1; + } else { + if (parent < ref->parent) + return -1; + if (parent > ref->parent) + return 1; + } + return 0; +} + +/* + * Check to see if a given root/parent reference is attached to the head. This + * only checks for BTRFS_ADD_DELAYED_REF references that match, as that + * indicates the reference exists for the given root or parent. This is for + * tree blocks only. + * + * @head: the head of the bytenr we're searching. + * @root: the root objectid of the reference if it is a normal reference. + * @parent: the parent if this is a shared backref. + */ +bool btrfs_find_delayed_tree_ref(struct btrfs_delayed_ref_head *head, + u64 root, u64 parent) +{ + struct rb_node *node; + bool found = false; + + lockdep_assert_held(&head->mutex); + + spin_lock(&head->lock); + node = head->ref_tree.rb_root.rb_node; + while (node) { + struct btrfs_delayed_ref_node *entry; + int ret; + + entry = rb_entry(node, struct btrfs_delayed_ref_node, ref_node); + ret = find_comp(entry, root, parent); + if (ret < 0) { + node = node->rb_left; + } else if (ret > 0) { + node = node->rb_right; + } else { + /* + * We only want to count ADD actions, as drops mean the + * ref doesn't exist. + */ + if (entry->action == BTRFS_ADD_DELAYED_REF) + found = true; + break; + } + } + spin_unlock(&head->lock); + return found; +} + void __cold btrfs_delayed_ref_exit(void) { kmem_cache_destroy(btrfs_delayed_ref_head_cachep); diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h index b291147cb8ab..34c34db01cc6 100644 --- a/fs/btrfs/delayed-ref.h +++ b/fs/btrfs/delayed-ref.h @@ -397,6 +397,8 @@ int btrfs_delayed_refs_rsv_refill(struct btrfs_fs_info *fs_info, void btrfs_migrate_to_delayed_refs_rsv(struct btrfs_fs_info *fs_info, u64 num_bytes); bool btrfs_check_space_for_delayed_refs(struct btrfs_fs_info *fs_info); +bool btrfs_find_delayed_tree_ref(struct btrfs_delayed_ref_head *head, + u64 root, u64 parent); /* * helper functions to cast a node into its container diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 42314604906a..b21998c1e33f 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -5428,23 +5428,62 @@ static int check_ref_exists(struct btrfs_trans_handle *trans, struct btrfs_root *root, u64 bytenr, u64 parent, int level) { + struct btrfs_delayed_ref_root *delayed_refs; + struct btrfs_delayed_ref_head *head; struct btrfs_path *path; struct btrfs_extent_inline_ref *iref; int ret; + bool exists = false; path = btrfs_alloc_path(); if (!path) return -ENOMEM; - +again: ret = lookup_extent_backref(trans, path, &iref, bytenr, root->fs_info->nodesize, parent, root->root_key.objectid, level, 0); + if (ret != -ENOENT) { + /* + * If we get 0 then we found our reference, return 1, else + * return the error if it's not -ENOENT; + */ + btrfs_free_path(path); + return (ret < 0 ) ? ret : 1; + } + + /* + * We could have a delayed ref with this reference, so look it up while + * we're holding the path open to make sure we don't race with the + * delayed ref running. + */ + delayed_refs = &trans->transaction->delayed_refs; + spin_lock(&delayed_refs->lock); + head = btrfs_find_delayed_ref_head(delayed_refs, bytenr); + if (!head) + goto out; + if (!mutex_trylock(&head->mutex)) { + /* + * We're contended, means that the delayed ref is running, get a + * reference and wait for the ref head to be complete and then + * try again. + */ + refcount_inc(&head->refs); + spin_unlock(&delayed_refs->lock); + + btrfs_release_path(path); + + mutex_lock(&head->mutex); + mutex_unlock(&head->mutex); + btrfs_put_delayed_ref_head(head); + goto again; + } + + exists = btrfs_find_delayed_tree_ref(head, root->root_key.objectid, parent); + mutex_unlock(&head->mutex); +out: + spin_unlock(&delayed_refs->lock); btrfs_free_path(path); - if (ret == -ENOENT) - return 0; - if (ret < 0) - return ret; - return 1; + return exists ? 1 : 0; } /*
In the patch 78c52d9eb6b7 ("btrfs: check for refs on snapshot delete resume") I added some code to handle file systems that had been corrupted by a bug that incorrectly skipped updating the drop progress key while dropping a snapshot. This code would check to see if we had already deleted our reference for a child block, and skip the deletion if we had already. Unfortunately there is a bug, as the check would only check the on-disk references. I made an incorrect assumption that blocks in an already deleted snapshot that was having the deletion resume on mount wouldn't be modified. If we have 2 pending deleted snapshots that share blocks, we can easily modify the rules for a block. Take the following example subvolume a exists, and subvolume b is a snapshot of subvolume a. They share references to block 1. Block 1 will have 2 full references, one for subvolume a and one for subvolume b, and it belongs to subvolume a (btrfs_header_owner(block 1) == subvolume a). When deleting subvolume a, we will drop our full reference for block 1, and because we are the owner we will drop our full reference for all of block 1's children, convert block 1 to FULL BACKREF, and add a shared reference to all of block 1's children. Then we will start the snapshot deletion of subvolume b. We look up the extent info for block 1, which checks delayed refs and tells us that FULL BACKREF is set, so sets parent to the bytenr of block 1. However because this is a resumed snapshot deletion, we call into check_ref_exists(). Because check_ref_exists() only looks at the disk, it doesn't find the shared backref for the child of block 1, and thus returns 0 and we skip deleting the reference for the child of block 1 and continue. This orphans the child of block 1. The fix is to lookup the delayed refs, similar to what we do in btrfs_lookup_extent_info(). However we only care about whether the reference exists or not. If we fail to find our reference on disk, go look up the bytenr in the delayed refs, and if it exists look for an existing ref in the delayed ref head. If that exists then we know we can delete the reference safely and carry on. If it doesn't exist we know we have to skip over this block. This bug has existed since I introduced this fix, however requires having multiple deleted snapshots pending when we unmount. We noticed this in production because our shutdown path stops the container on the system, which deletes a bunch of subvolumes, and then reboots the box. This gives us plenty of opportunities to hit this issue. Looking at the history we've seen this occasionally in production, but we had a big spike recently thanks to faster machines getting jobs with multiple subvolumes in the job. Chris Mason wrote a reproducer which does the following mount /dev/nvme4n1 /btrfs btrfs subvol create /btrfs/s1 simoop -E -f 4k -n 200000 -z /btrfs/s1 while(true) ; do btrfs subvol snap /btrfs/s1 /btrfs/s2 simoop -f 4k -n 200000 -r 10 -z /btrfs/s2 btrfs subvol snap /btrfs/s2 /btrfs/s3 btrfs balance start -dusage=80 /btrfs btrfs subvol del /btrfs/s2 /btrfs/s3 umount /btrfs btrfsck /dev/nvme4n1 || exit 1 mount /dev/nvme4n1 /btrfs done On the second loop this would fail consistently, with my patch it has been running for hours and hasn't failed. I also used dm-log-writes to capture the state of the failure so I could debug the problem. Using the existing failure case to test my patch validated that it fixes the problem. Fixes: 78c52d9eb6b7 ("btrfs: check for refs on snapshot delete resume") Signed-off-by: Josef Bacik <josef@toxicpanda.com> --- v1->v2: - Fixed up style nits pointed out by Filipe. - Fixed up the changelog to fix some wrong words and make things more clear. fs/btrfs/delayed-ref.c | 69 ++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/delayed-ref.h | 2 ++ fs/btrfs/extent-tree.c | 51 +++++++++++++++++++++++++++---- 3 files changed, 116 insertions(+), 6 deletions(-)