kernel/linux.git/fs/btrfs, branch v6.1.168

btrfs: do not free data reservation in fallback from inline due to -ENOSPC

2026-04-11T12:16:32+00:00

[ Upstream commit f8da41de0bff9eb1d774a7253da0c9f637c4470a ] If we fail to create an inline extent due to -ENOSPC, we will attempt to go through the normal COW path, reserve an extent, create an ordered extent, etc. However we were always freeing the reserved qgroup data, which is wrong since we will use data. Fix this by freeing the reserved qgroup data in __cow_file_range_inline() only if we are not doing the fallback (ret is <= 0). Reviewed-by: Qu Wenruo Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: fix the qgroup data free range for inline data extents

2026-04-11T12:16:32+00:00

[ Upstream commit 0bb067ca64e35536f1f5d9ef6aaafc40f4833623 ] Inside function __cow_file_range_inline() since the inlined data no longer take any data space, we need to free up the reserved space. However the code is still using the old page size == sector size assumption, and will not handle subpage case well. Thankfully it is not going to cause any problems because we have two extra safe nets: - Inline data extents creation is disabled for sector size < page size cases for now But it won't stay that for long. - btrfs_qgroup_free_data() will only clear ranges which have been already reserved So even if we pass a range larger than what we need, it should still be fine, especially there is only reserved space for a single block at file offset 0 of an inline data extent. But just for the sake of consistency, fix the call site to use sectorsize instead of page size. Reviewed-by: Filipe Manana Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba Stable-dep-of: f8da41de0bff ("btrfs: do not free data reservation in fallback from inline due to -ENOSPC") Signed-off-by: Sasha Levin

btrfs: reject root items with drop_progress and zero drop_level

2026-04-11T12:16:19+00:00

[ Upstream commit b17b79ff896305fd74980a5f72afec370ee88ca4 ] [BUG] When recovering relocation at mount time, merge_reloc_root() and btrfs_drop_snapshot() both use BUG_ON(level == 0) to guard against an impossible state: a non-zero drop_progress combined with a zero drop_level in a root_item, which can be triggered: ------------[ cut here ]------------ kernel BUG at fs/btrfs/relocation.c:1545! Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI CPU: 1 UID: 0 PID: 283 ... Tainted: 6.18.0+ #16 PREEMPT(voluntary) Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE Hardware name: QEMU Ubuntu 24.04 PC v2, BIOS 1.16.3-debian-1.16.3-2 RIP: 0010:merge_reloc_root+0x1266/0x1650 fs/btrfs/relocation.c:1545 Code: ffff0000 00004589 d7e9acfa ffffe8a1 79bafebe 02000000 Call Trace: merge_reloc_roots+0x295/0x890 fs/btrfs/relocation.c:1861 btrfs_recover_relocation+0xd6e/0x11d0 fs/btrfs/relocation.c:4195 btrfs_start_pre_rw_mount+0xa4d/0x1810 fs/btrfs/disk-io.c:3130 open_ctree+0x5824/0x5fe0 fs/btrfs/disk-io.c:3640 btrfs_fill_super fs/btrfs/super.c:987 [inline] btrfs_get_tree_super fs/btrfs/super.c:1951 [inline] btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline] btrfs_get_tree+0x111c/0x2190 fs/btrfs/super.c:2128 vfs_get_tree+0x9a/0x370 fs/super.c:1758 fc_mount fs/namespace.c:1199 [inline] do_new_mount_fc fs/namespace.c:3642 [inline] do_new_mount fs/namespace.c:3718 [inline] path_mount+0x5b8/0x1ea0 fs/namespace.c:4028 do_mount fs/namespace.c:4041 [inline] __do_sys_mount fs/namespace.c:4229 [inline] __se_sys_mount fs/namespace.c:4206 [inline] __x64_sys_mount+0x282/0x320 fs/namespace.c:4206 ... RIP: 0033:0x7f969c9a8fde Code: 0f1f4000 48c7c2b0 fffffff7 d8648902 b8ffffff ffc3660f ---[ end trace 0000000000000000 ]--- The bug is reproducible on 7.0.0-rc2-next-20260310 with our dynamic metadata fuzzing tool that corrupts btrfs metadata at runtime. [CAUSE] A non-zero drop_progress.objectid means an interrupted btrfs_drop_snapshot() left a resume point on disk, and in that case drop_level must be greater than 0 because the checkpoint is only saved at internal node levels. Although this invariant is enforced when the kernel writes the root item, it is not validated when the root item is read back from disk. That allows on-disk corruption to provide an invalid state with drop_progress.objectid != 0 and drop_level == 0. When relocation recovery later processes such a root item, merge_reloc_root() reads drop_level and hits BUG_ON(level == 0). The same invalid metadata can also trigger the corresponding BUG_ON() in btrfs_drop_snapshot(). [FIX] Fix this by validating the root_item invariant in tree-checker when reading root items from disk: if drop_progress.objectid is non-zero, drop_level must also be non-zero. Reject such malformed metadata with -EUCLEAN before it reaches merge_reloc_root() or btrfs_drop_snapshot() and triggers the BUG_ON. After the fix, the same corruption is correctly rejected by tree-checker and the BUG_ON is no longer triggered. Reviewed-by: Qu Wenruo Signed-off-by: ZhengYuan Huang Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: don't take device_list_mutex when querying zone info

2026-04-11T12:16:18+00:00

[ Upstream commit 77603ab10429fe713a03345553ca8dbbfb1d91c6 ] Shin'ichiro reported sporadic hangs when running generic/013 in our CI system. When enabling lockdep, there is a lockdep splat when calling btrfs_get_dev_zone_info_all_devices() in the mount path that can be triggered by i.e. generic/013: ====================================================== WARNING: possible circular locking dependency detected 7.0.0-rc1+ #355 Not tainted ------------------------------------------------------ mount/1043 is trying to acquire lock: ffff8881020b5470 (&vblk->vdev_mutex){+.+.}-{4:4}, at: virtblk_report_zones+0xda/0x430 but task is already holding lock: ffff888102a738e0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_get_dev_zone_info_all_devices+0x45/0x90 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #4 (&fs_devs->device_list_mutex){+.+.}-{4:4}: __mutex_lock+0xa3/0x1360 btrfs_create_pending_block_groups+0x1f4/0x9d0 __btrfs_end_transaction+0x3e/0x2e0 btrfs_zoned_reserve_data_reloc_bg+0x2f8/0x390 open_ctree+0x1934/0x23db btrfs_get_tree.cold+0x105/0x26c vfs_get_tree+0x28/0xb0 __do_sys_fsconfig+0x324/0x680 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #3 (btrfs_trans_num_extwriters){++++}-{0:0}: join_transaction+0xc2/0x5c0 start_transaction+0x17c/0xbc0 btrfs_zoned_reserve_data_reloc_bg+0x2b4/0x390 open_ctree+0x1934/0x23db btrfs_get_tree.cold+0x105/0x26c vfs_get_tree+0x28/0xb0 __do_sys_fsconfig+0x324/0x680 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #2 (btrfs_trans_num_writers){++++}-{0:0}: lock_release+0x163/0x4b0 __btrfs_end_transaction+0x1c7/0x2e0 btrfs_dirty_inode+0x6f/0xd0 touch_atime+0xe5/0x2c0 btrfs_file_mmap_prepare+0x65/0x90 __mmap_region+0x4b9/0xf00 mmap_region+0xf7/0x120 do_mmap+0x43d/0x610 vm_mmap_pgoff+0xd6/0x190 ksys_mmap_pgoff+0x7e/0xc0 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #1 (&mm->mmap_lock){++++}-{4:4}: __might_fault+0x68/0xa0 _copy_to_user+0x22/0x70 blkdev_copy_zone_to_user+0x22/0x40 virtblk_report_zones+0x282/0x430 blkdev_report_zones_ioctl+0xfd/0x130 blkdev_ioctl+0x20f/0x2c0 __x64_sys_ioctl+0x86/0xd0 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e -> #0 (&vblk->vdev_mutex){+.+.}-{4:4}: __lock_acquire+0x1522/0x2680 lock_acquire+0xd5/0x2f0 __mutex_lock+0xa3/0x1360 virtblk_report_zones+0xda/0x430 blkdev_report_zones_cached+0x162/0x190 btrfs_get_dev_zones+0xdc/0x2e0 btrfs_get_dev_zone_info+0x219/0xe80 btrfs_get_dev_zone_info_all_devices+0x62/0x90 open_ctree+0x1200/0x23db btrfs_get_tree.cold+0x105/0x26c vfs_get_tree+0x28/0xb0 __do_sys_fsconfig+0x324/0x680 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e other info that might help us debug this: Chain exists of: &vblk->vdev_mutex --> btrfs_trans_num_extwriters --> &fs_devs->device_list_mutex Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&fs_devs->device_list_mutex); lock(btrfs_trans_num_extwriters); lock(&fs_devs->device_list_mutex); lock(&vblk->vdev_mutex); *** DEADLOCK *** 3 locks held by mount/1043: #0: ffff88811063e878 (&fc->uapi_mutex){+.+.}-{4:4}, at: __do_sys_fsconfig+0x2ae/0x680 #1: ffff88810cb9f0e8 (&type->s_umount_key#31/1){+.+.}-{4:4}, at: alloc_super+0xc0/0x3e0 #2: ffff888102a738e0 (&fs_devs->device_list_mutex){+.+.}-{4:4}, at: btrfs_get_dev_zone_info_all_devices+0x45/0x90 stack backtrace: CPU: 2 UID: 0 PID: 1043 Comm: mount Not tainted 7.0.0-rc1+ #355 PREEMPT(full) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025 Call Trace: dump_stack_lvl+0x5b/0x80 print_circular_bug.cold+0x18d/0x1d8 check_noncircular+0x10d/0x130 __lock_acquire+0x1522/0x2680 ? vmap_small_pages_range_noflush+0x3ef/0x820 lock_acquire+0xd5/0x2f0 ? virtblk_report_zones+0xda/0x430 ? lock_is_held_type+0xcd/0x130 __mutex_lock+0xa3/0x1360 ? virtblk_report_zones+0xda/0x430 ? virtblk_report_zones+0xda/0x430 ? __pfx_copy_zone_info_cb+0x10/0x10 ? virtblk_report_zones+0xda/0x430 virtblk_report_zones+0xda/0x430 ? __pfx_copy_zone_info_cb+0x10/0x10 blkdev_report_zones_cached+0x162/0x190 ? __pfx_copy_zone_info_cb+0x10/0x10 btrfs_get_dev_zones+0xdc/0x2e0 btrfs_get_dev_zone_info+0x219/0xe80 btrfs_get_dev_zone_info_all_devices+0x62/0x90 open_ctree+0x1200/0x23db btrfs_get_tree.cold+0x105/0x26c ? rcu_is_watching+0x18/0x50 vfs_get_tree+0x28/0xb0 __do_sys_fsconfig+0x324/0x680 do_syscall_64+0x92/0x4f0 entry_SYSCALL_64_after_hwframe+0x76/0x7e RIP: 0033:0x7f615e27a40e RSP: 002b:00007fff11b18fb8 EFLAGS: 00000246 ORIG_RAX: 00000000000001af RAX: ffffffffffffffda RBX: 000055572e92ab10 RCX: 00007f615e27a40e RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003 RBP: 00007fff11b19100 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 000055572e92bc40 R14: 00007f615e3faa60 R15: 000055572e92bd08 Don't hold the device_list_mutex while calling into btrfs_get_dev_zone_info() in btrfs_get_dev_zone_info_all_devices() to mitigate the issue. This is safe, as no other thread can touch the device list at the moment of execution. Reported-by: Shin'ichiro Kawasaki Reviewed-by: Damien Le Moal Signed-off-by: Johannes Thumshirn Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: fix lost error when running device stats on multiple devices fs

2026-04-11T12:16:17+00:00

[ Upstream commit 1c37d896b12dfd0d4c96e310b0033c6676933917 ] Whenever we get an error updating the device stats item for a device in btrfs_run_dev_stats() we allow the loop to go to the next device, and if updating the stats item for the next device succeeds, we end up losing the error we had from the previous device. Fix this by breaking out of the loop once we get an error and make sure it's returned to the caller. Since we are in the transaction commit path (and in the critical section actually), returning the error will result in a transaction abort. Fixes: 733f4fbbc108 ("Btrfs: read device stats on mount, write modified ones during commit") Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: fix leak of kobject name for sub-group space_info

2026-04-11T12:16:17+00:00

[ Upstream commit a4376d9a5d4c9610e69def3fc0b32c86a7ab7a41 ] When create_space_info_sub_group() allocates elements of space_info->sub_group[], kobject_init_and_add() is called for each element via btrfs_sysfs_add_space_info_type(). However, when check_removing_space_info() frees these elements, it does not call btrfs_sysfs_remove_space_info() on them. As a result, kobject_put() is not called and the associated kobj->name objects are leaked. This memory leak is reproduced by running the blktests test case zbd/009 on kernels built with CONFIG_DEBUG_KMEMLEAK. The kmemleak feature reports the following error: unreferenced object 0xffff888112877d40 (size 16): comm "mount", pid 1244, jiffies 4294996972 hex dump (first 16 bytes): 64 61 74 61 2d 72 65 6c 6f 63 00 c4 c6 a7 cb 7f data-reloc...... backtrace (crc 53ffde4d): __kmalloc_node_track_caller_noprof+0x619/0x870 kstrdup+0x42/0xc0 kobject_set_name_vargs+0x44/0x110 kobject_init_and_add+0xcf/0x150 btrfs_sysfs_add_space_info_type+0xfc/0x210 [btrfs] create_space_info_sub_group.constprop.0+0xfb/0x1b0 [btrfs] create_space_info+0x211/0x320 [btrfs] btrfs_init_space_info+0x15a/0x1b0 [btrfs] open_ctree+0x33c7/0x4a50 [btrfs] btrfs_get_tree.cold+0x9f/0x1ee [btrfs] vfs_get_tree+0x87/0x2f0 vfs_cmd_create+0xbd/0x280 __do_sys_fsconfig+0x3df/0x990 do_syscall_64+0x136/0x1540 entry_SYSCALL_64_after_hwframe+0x76/0x7e To avoid the leak, call btrfs_sysfs_remove_space_info() instead of kfree() for the elements. Fixes: f92ee31e031c ("btrfs: introduce btrfs_space_info sub-group") Link: https://lore.kernel.org/linux-block/b9488881-f18d-4f47-91a5-3c9bf63955a5@wdc.com/ Reviewed-by: Johannes Thumshirn Signed-off-by: Shin'ichiro Kawasaki Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: fix super block offset in error message in btrfs_validate_super()

2026-04-11T12:16:17+00:00

[ Upstream commit b52fe51f724385b3ed81e37e510a4a33107e8161 ] Fix the superblock offset mismatch error message in btrfs_validate_super(): we changed it so that it considers all the superblocks, but the message still assumes we're only looking at the first one. The change from %u to %llu is because we're changing from a constant to a u64. Fixes: 069ec957c35e ("btrfs: Refactor btrfs_check_super_valid") Reviewed-by: Qu Wenruo Signed-off-by: Mark Harmstone Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: set BTRFS_ROOT_ORPHAN_CLEANUP during subvol create

2026-04-11T12:16:04+00:00

[ Upstream commit 5131fa077f9bb386a1b901bf5b247041f0ec8f80 ] We have recently observed a number of subvolumes with broken dentries. ls-ing the parent dir looks like: drwxrwxrwt 1 root root 16 Jan 23 16:49 . drwxr-xr-x 1 root root 24 Jan 23 16:48 .. d????????? ? ? ? ? ? broken_subvol and similarly stat-ing the file fails. In this state, deleting the subvol fails with ENOENT, but attempting to create a new file or subvol over it errors out with EEXIST and even aborts the fs. Which leaves us a bit stuck. dmesg contains a single notable error message reading: "could not do orphan cleanup -2" 2 is ENOENT and the error comes from the failure handling path of btrfs_orphan_cleanup(), with the stack leading back up to btrfs_lookup(). btrfs_lookup btrfs_lookup_dentry btrfs_orphan_cleanup // prints that message and returns -ENOENT After some detailed inspection of the internal state, it became clear that: - there are no orphan items for the subvol - the subvol is otherwise healthy looking, it is not half-deleted or anything, there is no drop progress, etc. - the subvol was created a while ago and does the meaningful first btrfs_orphan_cleanup() call that sets BTRFS_ROOT_ORPHAN_CLEANUP much later. - after btrfs_orphan_cleanup() fails, btrfs_lookup_dentry() returns -ENOENT, which results in a negative dentry for the subvolume via d_splice_alias(NULL, dentry), leading to the observed behavior. The bug can be mitigated by dropping the dentry cache, at which point we can successfully delete the subvolume if we want. i.e., btrfs_lookup() btrfs_lookup_dentry() if (!sb_rdonly(inode->vfs_inode)->vfs_inode) btrfs_orphan_cleanup(sub_root) test_and_set_bit(BTRFS_ROOT_ORPHAN_CLEANUP) btrfs_search_slot() // finds orphan item for inode N ... prints "could not do orphan cleanup -2" if (inode == ERR_PTR(-ENOENT)) inode = NULL; return d_splice_alias(NULL, dentry) // NEGATIVE DENTRY for valid subvolume btrfs_orphan_cleanup() does test_and_set_bit(BTRFS_ROOT_ORPHAN_CLEANUP) on the root when it runs, so it cannot run more than once on a given root, so something else must run concurrently. However, the obvious routes to deleting an orphan when nlinks goes to 0 should not be able to run without first doing a lookup into the subvolume, which should run btrfs_orphan_cleanup() and set the bit. The final important observation is that create_subvol() calls d_instantiate_new() but does not set BTRFS_ROOT_ORPHAN_CLEANUP, so if the dentry cache gets dropped, the next lookup into the subvolume will make a real call into btrfs_orphan_cleanup() for the first time. This opens up the possibility of concurrently deleting the inode/orphan items but most typical evict() paths will be holding a reference on the parent dentry (child dentry holds parent->d_lockref.count via dget in d_alloc(), released in __dentry_kill()) and prevent the parent from being removed from the dentry cache. The one exception is delayed iputs. Ordered extent creation calls igrab() on the inode. If the file is unlinked and closed while those refs are held, iput() in __dentry_kill() decrements i_count but does not trigger eviction (i_count > 0). The child dentry is freed and the subvol dentry's d_lockref.count drops to 0, making it evictable while the inode is still alive. Since there are two races (the race between writeback and unlink and the race between lookup and delayed iputs), and there are too many moving parts, the following three diagrams show the complete picture. (Only the second and third are races) Phase 1: Create Subvol in dentry cache without BTRFS_ROOT_ORPHAN_CLEANUP set btrfs_mksubvol() lookup_one_len() __lookup_slow() d_alloc_parallel() __d_alloc() // d_lockref.count = 1 create_subvol(dentry) // doesn't touch the bit.. d_instantiate_new(dentry, inode) // dentry in cache with d_lockref.count == 1 Phase 2: Create a delayed iput for a file in the subvol but leave the subvol in state where its dentry can be evicted (d_lockref.count == 0) T1 (task) T2 (writeback) T3 (OE workqueue) write() // dirty pages btrfs_writepages() btrfs_run_delalloc_range() cow_file_range() btrfs_alloc_ordered_extent() igrab() // i_count: 1 -> 2 btrfs_unlink_inode() btrfs_orphan_add() close() __fput() dput() finish_dput() __dentry_kill() dentry_unlink_inode() iput() // 2 -> 1 --parent->d_lockref.count // 1 -> 0; evictable finish_ordered_fn() btrfs_finish_ordered_io() btrfs_put_ordered_extent() btrfs_add_delayed_iput() Phase 3: Once the delayed iput is pending and the subvol dentry is evictable, the shrinker can free it, causing the next lookup to go through btrfs_lookup() and call btrfs_orphan_cleanup() for the first time. If the cleaner kthread processes the delayed iput concurrently, the two race: T1 (shrinker) T2 (cleaner kthread) T3 (lookup) super_cache_scan() prune_dcache_sb() __dentry_kill() // subvol dentry freed btrfs_run_delayed_iputs() iput() // i_count -> 0 evict() // sets I_FREEING btrfs_evict_inode() // truncation loop btrfs_lookup() btrfs_lookup_dentry() btrfs_orphan_cleanup() // first call (bit never set) btrfs_iget() // blocks on I_FREEING btrfs_orphan_del() // inode freed // returns -ENOENT btrfs_del_orphan_item() // -ENOENT // "could not do orphan cleanup -2" d_splice_alias(NULL, dentry) // negative dentry for valid subvol The most straightforward fix is to ensure the invariant that a dentry for a subvolume can exist if and only if that subvolume has BTRFS_ROOT_ORPHAN_CLEANUP set on its root (and is known to have no orphans or ran btrfs_orphan_cleanup()). Reviewed-by: Filipe Manana Signed-off-by: Boris Burkov Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: tree-checker: fix misleading root drop_level error message

2026-03-25T10:03:20+00:00

[ Upstream commit fc1cd1f18c34f91e78362f9629ab9fd43b9dcab9 ] Fix tree-checker error message to report "invalid root drop_level" instead of the misleading "invalid root level". Fixes: 259ee7754b67 ("btrfs: tree-checker: Add ROOT_ITEM check") Reviewed-by: Qu Wenruo Signed-off-by: ZhengYuan Huang Reviewed-by: David Sterba Signed-off-by: David Sterba Signed-off-by: Sasha Levin

btrfs: do not strictly require dirty metadata threshold for metadata writepages

2026-03-25T10:03:19+00:00

[ Upstream commit 4e159150a9a56d66d247f4b5510bed46fe58aa1c ] [BUG] There is an internal report that over 1000 processes are waiting at the io_schedule_timeout() of balance_dirty_pages(), causing a system hang and trigger a kernel coredump. The kernel is v6.4 kernel based, but the root problem still applies to any upstream kernel before v6.18. [CAUSE] >From Jan Kara for his wisdom on the dirty page balance behavior first. This cgroup dirty limit was what was actually playing the role here because the cgroup had only a small amount of memory and so the dirty limit for it was something like 16MB. Dirty throttling is responsible for enforcing that nobody can dirty (significantly) more dirty memory than there's dirty limit. Thus when a task is dirtying pages it periodically enters into balance_dirty_pages() and we let it sleep there to slow down the dirtying. When the system is over dirty limit already (either globally or within a cgroup of the running task), we will not let the task exit from balance_dirty_pages() until the number of dirty pages drops below the limit. So in this particular case, as I already mentioned, there was a cgroup with relatively small amount of memory and as a result with dirty limit set at 16MB. A task from that cgroup has dirtied about 28MB worth of pages in btrfs btree inode and these were practically the only dirty pages in that cgroup. So that means the only way to reduce the dirty pages of that cgroup is to writeback the dirty pages of btrfs btree inode, and only after that those processes can exit balance_dirty_pages(). Now back to the btrfs part, btree_writepages() is responsible for writing back dirty btree inode pages. The problem here is, there is a btrfs internal threshold that if the btree inode's dirty bytes are below the 32M threshold, it will not do any writeback. This behavior is to batch as much metadata as possible so we won't write back those tree blocks and then later re-COW them again for another modification. This internal 32MiB is higher than the existing dirty page size (28MiB), meaning no writeback will happen, causing a deadlock between btrfs and cgroup: - Btrfs doesn't want to write back btree inode until more dirty pages - Cgroup/MM doesn't want more dirty pages for btrfs btree inode Thus any process touching that btree inode is put into sleep until the number of dirty pages is reduced. Thanks Jan Kara a lot for the analysis of the root cause. [ENHANCEMENT] Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on the btree_inode"), btrfs btree inode pages will only be charged to the root cgroup which should have a much larger limit than btrfs' 32MiB threshold. So it should not affect newer kernels. But for all current LTS kernels, they are all affected by this problem, and backporting the whole AS_KERNEL_FILE may not be a good idea. Even for newer kernels I still think it's a good idea to get rid of the internal threshold at btree_writepages(), since for most cases cgroup/MM has a better view of full system memory usage than btrfs' fixed threshold. For internal callers using btrfs_btree_balance_dirty() since that function is already doing internal threshold check, we don't need to bother them. But for external callers of btree_writepages(), just respect their requests and write back whatever they want, ignoring the internal btrfs threshold to avoid such deadlock on btree inode dirty page balancing. CC: stable@vger.kernel.org CC: Jan Kara Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Signed-off-by: David Sterba [ The context change is due to the commit 41044b41ad2c ("btrfs: add helper to get fs_info from struct inode pointer") in v6.9 and the commit c66f2afc7148 ("btrfs: remove pointless writepages callback wrapper") in v6.10 which are irrelevant to the logic of this patch. ] Signed-off-by: Rahul Sharma Signed-off-by: Greg Kroah-Hartman