| Message ID | 04478f1919b69f470f1c56bae80512491c97a8b1.1729277554.git.boris@bur.io (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | btrfs: hold merge refcount during extent_map merge | expand |
在 2024/10/19 06:17, Boris Burkov 写道: > In debugging some corrupt squashfs files, we observed symptoms of > corrupt page cache pages but correct on-disk contents. Further > investigation revealed that the exact symptom was a correct page > followed by an incorrect, duplicate, page. This got us thinking about > extent maps. > > commit ac05ca913e9f ("Btrfs: fix race between using extent maps and merging them") > enforces a reference count on the primary `em` extent_map being merged, > as that one gets modified. Indeed, at that time @merge is read-only then removed from the rb tree. No modification to the @merge members. > > However, since, > commit 3d2ac9922465 ("btrfs: introduce new members for extent_map") > both 'em' and 'merge' get modified, which introduced the same race and > need for refcounting for 'merge'. But I changed that, to make both @merge and @em to be the same, initially I thought it's safer but it's the completely the opposite. > > We were able to reproduce this by looping the affected squashfs workload > in parallel on a bunch of separate btrfs-es while also dropping caches. > > The fix is two-fold: > - check the refs > 2 criterion for both 'em' and 'merge' > - actually refcount 'merge' which we currently grab without a refcount > via rb_prev or rb_next. > > The other option we considered, but did not implement, was to stop > modifying 'merge' as it gets deleted immediately after. However, it was > less clear what effect that would have on the racing thread holding the > reference, so we went with this fix. > > Fixes: 3d2ac9922465 ("btrfs: introduce new members for extent_map") > Signed-off-by: Omar Sandoval <osandov@fb.com> > Signed-off-by: Boris Burkov <boris@bur.io> Reviewed-by: Qu Wenruo <wqu@suse.com> Thanks a lot for pinning down the bug and fix, Qu > --- > fs/btrfs/extent_map.c | 92 +++++++++++++++++++++++++++---------------- > 1 file changed, 57 insertions(+), 35 deletions(-) > > diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c > index d58d6fc40da1..108cd7573f61 100644 > --- a/fs/btrfs/extent_map.c > +++ b/fs/btrfs/extent_map.c > @@ -215,6 +215,16 @@ static bool can_merge_extent_map(const struct extent_map *em) > > if (em->flags & EXTENT_FLAG_LOGGING) > return false; > + /* > + * We can't modify an extent map that is in the tree and that is being > + * used by another task, as it can cause that other task to see it in > + * inconsistent state during the merging. We always have 1 reference for > + * the tree and 1 for this task (which is unpinning the extent map or > + * clearing the logging flag), so anything > 2 means it's being used by > + * other tasks too. > + */ > + if (refcount_read(&em->refs) > 2) > + return false; > > /* > * We don't want to merge stuff that hasn't been written to the log yet > @@ -333,57 +343,69 @@ static void validate_extent_map(struct btrfs_fs_info *fs_info, struct extent_map > } > } > > -static void try_merge_map(struct btrfs_inode *inode, struct extent_map *em) > +static int try_merge_one_map(struct btrfs_inode *inode, struct extent_map *em, > + struct extent_map *merge, bool prev_merge) > { > struct btrfs_fs_info *fs_info = inode->root->fs_info; > - struct extent_map *merge = NULL; > - struct rb_node *rb; > + struct extent_map *prev; > + struct extent_map *next; > > + if (prev_merge) { > + prev = merge; > + next = em; > + } else { > + prev = em; > + next = merge; > + } > /* > - * We can't modify an extent map that is in the tree and that is being > - * used by another task, as it can cause that other task to see it in > - * inconsistent state during the merging. We always have 1 reference for > - * the tree and 1 for this task (which is unpinning the extent map or > - * clearing the logging flag), so anything > 2 means it's being used by > - * other tasks too. > - */ > - if (refcount_read(&em->refs) > 2) > - return; > + * Take a refcount for merge so we can maintain a proper > + * use count across threads for checking mergeability. > + */ > + refcount_inc(&merge->refs); > + if (!can_merge_extent_map(merge)) > + goto no_merge; > + > + if (!mergeable_maps(prev, next)) > + goto no_merge; > + > + if (prev_merge) > + em->start = merge->start; > + em->len += merge->len; > + em->generation = max(em->generation, merge->generation); > + if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) > + merge_ondisk_extents(prev, next); > + em->flags |= EXTENT_FLAG_MERGED; > + > + validate_extent_map(fs_info, em); > + refcount_dec(&merge->refs); > + remove_em(inode, merge); > + free_extent_map(merge); > + return 0; > +no_merge: > + refcount_dec(&merge->refs); > + return 1; > +} > + > +static void try_merge_map(struct btrfs_inode *inode, struct extent_map *em) > +{ > + struct extent_map *merge = NULL; > + struct rb_node *rb; > > if (!can_merge_extent_map(em)) > return; > > if (em->start != 0) { > rb = rb_prev(&em->rb_node); > - if (rb) > + if (rb) { > merge = rb_entry(rb, struct extent_map, rb_node); > - if (rb && can_merge_extent_map(merge) && mergeable_maps(merge, em)) { > - em->start = merge->start; > - em->len += merge->len; > - em->generation = max(em->generation, merge->generation); > - > - if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) > - merge_ondisk_extents(merge, em); > - em->flags |= EXTENT_FLAG_MERGED; > - > - validate_extent_map(fs_info, em); > - remove_em(inode, merge); > - free_extent_map(merge); > + try_merge_one_map(inode, em, merge, true); > } > } > > rb = rb_next(&em->rb_node); > - if (rb) > + if (rb) { > merge = rb_entry(rb, struct extent_map, rb_node); > - if (rb && can_merge_extent_map(merge) && mergeable_maps(em, merge)) { > - em->len += merge->len; > - if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) > - merge_ondisk_extents(em, merge); > - validate_extent_map(fs_info, em); > - em->generation = max(em->generation, merge->generation); > - em->flags |= EXTENT_FLAG_MERGED; > - remove_em(inode, merge); > - free_extent_map(merge); > + try_merge_one_map(inode, em, merge, false); > } > } >
diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c index d58d6fc40da1..108cd7573f61 100644 --- a/fs/btrfs/extent_map.c +++ b/fs/btrfs/extent_map.c @@ -215,6 +215,16 @@ static bool can_merge_extent_map(const struct extent_map *em) if (em->flags & EXTENT_FLAG_LOGGING) return false; + /* + * We can't modify an extent map that is in the tree and that is being + * used by another task, as it can cause that other task to see it in + * inconsistent state during the merging. We always have 1 reference for + * the tree and 1 for this task (which is unpinning the extent map or + * clearing the logging flag), so anything > 2 means it's being used by + * other tasks too. + */ + if (refcount_read(&em->refs) > 2) + return false; /* * We don't want to merge stuff that hasn't been written to the log yet @@ -333,57 +343,69 @@ static void validate_extent_map(struct btrfs_fs_info *fs_info, struct extent_map } } -static void try_merge_map(struct btrfs_inode *inode, struct extent_map *em) +static int try_merge_one_map(struct btrfs_inode *inode, struct extent_map *em, + struct extent_map *merge, bool prev_merge) { struct btrfs_fs_info *fs_info = inode->root->fs_info; - struct extent_map *merge = NULL; - struct rb_node *rb; + struct extent_map *prev; + struct extent_map *next; + if (prev_merge) { + prev = merge; + next = em; + } else { + prev = em; + next = merge; + } /* - * We can't modify an extent map that is in the tree and that is being - * used by another task, as it can cause that other task to see it in - * inconsistent state during the merging. We always have 1 reference for - * the tree and 1 for this task (which is unpinning the extent map or - * clearing the logging flag), so anything > 2 means it's being used by - * other tasks too. - */ - if (refcount_read(&em->refs) > 2) - return; + * Take a refcount for merge so we can maintain a proper + * use count across threads for checking mergeability. + */ + refcount_inc(&merge->refs); + if (!can_merge_extent_map(merge)) + goto no_merge; + + if (!mergeable_maps(prev, next)) + goto no_merge; + + if (prev_merge) + em->start = merge->start; + em->len += merge->len; + em->generation = max(em->generation, merge->generation); + if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) + merge_ondisk_extents(prev, next); + em->flags |= EXTENT_FLAG_MERGED; + + validate_extent_map(fs_info, em); + refcount_dec(&merge->refs); + remove_em(inode, merge); + free_extent_map(merge); + return 0; +no_merge: + refcount_dec(&merge->refs); + return 1; +} + +static void try_merge_map(struct btrfs_inode *inode, struct extent_map *em) +{ + struct extent_map *merge = NULL; + struct rb_node *rb; if (!can_merge_extent_map(em)) return; if (em->start != 0) { rb = rb_prev(&em->rb_node); - if (rb) + if (rb) { merge = rb_entry(rb, struct extent_map, rb_node); - if (rb && can_merge_extent_map(merge) && mergeable_maps(merge, em)) { - em->start = merge->start; - em->len += merge->len; - em->generation = max(em->generation, merge->generation); - - if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) - merge_ondisk_extents(merge, em); - em->flags |= EXTENT_FLAG_MERGED; - - validate_extent_map(fs_info, em); - remove_em(inode, merge); - free_extent_map(merge); + try_merge_one_map(inode, em, merge, true); } } rb = rb_next(&em->rb_node); - if (rb) + if (rb) { merge = rb_entry(rb, struct extent_map, rb_node); - if (rb && can_merge_extent_map(merge) && mergeable_maps(em, merge)) { - em->len += merge->len; - if (em->disk_bytenr < EXTENT_MAP_LAST_BYTE) - merge_ondisk_extents(em, merge); - validate_extent_map(fs_info, em); - em->generation = max(em->generation, merge->generation); - em->flags |= EXTENT_FLAG_MERGED; - remove_em(inode, merge); - free_extent_map(merge); + try_merge_one_map(inode, em, merge, false); } }
In debugging some corrupt squashfs files, we observed symptoms of corrupt page cache pages but correct on-disk contents. Further investigation revealed that the exact symptom was a correct page followed by an incorrect, duplicate, page. This got us thinking about extent maps. commit ac05ca913e9f ("Btrfs: fix race between using extent maps and merging them") enforces a reference count on the primary `em` extent_map being merged, as that one gets modified. However, since, commit 3d2ac9922465 ("btrfs: introduce new members for extent_map") both 'em' and 'merge' get modified, which introduced the same race and need for refcounting for 'merge'. We were able to reproduce this by looping the affected squashfs workload in parallel on a bunch of separate btrfs-es while also dropping caches. The fix is two-fold: - check the refs > 2 criterion for both 'em' and 'merge' - actually refcount 'merge' which we currently grab without a refcount via rb_prev or rb_next. The other option we considered, but did not implement, was to stop modifying 'merge' as it gets deleted immediately after. However, it was less clear what effect that would have on the racing thread holding the reference, so we went with this fix. Fixes: 3d2ac9922465 ("btrfs: introduce new members for extent_map") Signed-off-by: Omar Sandoval <osandov@fb.com> Signed-off-by: Boris Burkov <boris@bur.io> --- fs/btrfs/extent_map.c | 92 +++++++++++++++++++++++++++---------------- 1 file changed, 57 insertions(+), 35 deletions(-)