kernel/linux.git/fs/fs-writeback.c, branch v5.10.261

writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

2026-07-24T13:49:21+00:00

[ Upstream commit cba38ec4cbd3a7b8b942a8d52531a05be8a9ff0d ] When a container exits, the following BUG_ON() is occasionally triggered: ================================================================== VFS: Busy inodes after unmount of sdb (ext4) ------------[ cut here ]------------ kernel BUG at fs/super.c:695! CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1 pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) pc : generic_shutdown_super+0xf0/0x100 lr : generic_shutdown_super+0xf0/0x100 Call trace: generic_shutdown_super+0xf0/0x100 kill_block_super+0x20/0x48 ext4_kill_sb+0x28/0x60 deactivate_locked_super+0x54/0x130 deactivate_super+0x84/0xa0 cleanup_mnt+0xa4/0x140 __cleanup_mnt+0x18/0x28 task_work_run+0x78/0xe0 do_notify_resume+0x204/0x240 ================================================================== The root cause is a race between cgroup_writeback_umount() and inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between inode_prepare_wbs_switch() returning true and the subsequent wb_queue_isw() call. Following is the process that triggers the issue: CPU A (umount) | CPU B (writeback) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ inode_switch_wbs/cleanup_offline_cgwb atomic_inc(&isw_nr_in_flight) inode_prepare_wbs_switch -> passes SB_ACTIVE check __iget(inode) generic_shutdown_super sb->s_flags &= ~SB_ACTIVE cgroup_writeback_umount(sb) smp_mb() atomic_read(&isw_nr_in_flight) rcu_barrier() -> no pending RCU callbacks flush_workqueue(isw_wq) -> nothing queued, returns evict_inodes(sb) -> Inode skipped as isw still holds a ref. sop->put_super(sb) /* destroys percpu counters */ -> VFS: Busy inodes after unmount! wb_queue_isw() queue_work(isw_wq, ...) /* later in work function */ inode_switch_wbs_work_fn process_inode_switch_wbs iput() -> evict percpu_counter_dec() // UAF! Fix this by extending the RCU read-side critical section in inode_switch_wbs() and cleanup_offline_cgwb() to cover from inode_prepare_wbs_switch() through wb_queue_isw(). Since there is no sleep in this window, rcu_read_lock() can be used. Then add a synchronize_rcu() in cgroup_writeback_umount() before the existing rcu_barrier(), so that all in-flight switchers that have passed the SB_ACTIVE check have completed queue_work() before flush_workqueue() is called. The existing rcu_barrier() is intentionally retained so this fix can be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that still queue switches via queue_rcu_work(). It is a no-op on current mainline (since commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")) and is removed in a follow-up patch. Fixes: a1a0e23e4903 ("writeback: flush inode cgroup wb switches instead of pinning super_block") Cc: stable@vger.kernel.org Suggested-by: Jan Kara Link: https://lore.kernel.org/all/mxnjq2l6guusfchvauxr3v7c4bwjasybxlleqbbh4efloeqspz@iqylk76ohufz Reviewed-by: Jan Kara Signed-off-by: Baokun Li Link: https://patch.msgid.link/20260521095016.2791354-2-libaokun@linux.alibaba.com Acked-by: Tejun Heo Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

writeback: fix 100% CPU usage when dirtytime_expire_interval is 0

2026-02-06T15:40:14+00:00

[ Upstream commit 543467d6fe97e27e22a26e367fda972dbefebbff ] When vm.dirtytime_expire_seconds is set to 0, wakeup_dirtytime_writeback() schedules delayed work with a delay of 0, causing immediate execution. The function then reschedules itself with 0 delay again, creating an infinite busy loop that causes 100% kworker CPU usage. Fix by: - Only scheduling delayed work in wakeup_dirtytime_writeback() when dirtytime_expire_interval is non-zero - Cancelling the delayed work in dirtytime_interval_handler() when the interval is set to 0 - Adding a guard in start_dirtytime_writeback() for defensive coding Tested by booting kernel in QEMU with virtme-ng: - Before fix: kworker CPU spikes to ~73% - After fix: CPU remains at normal levels - Setting interval back to non-zero correctly resumes writeback Fixes: a2f4870697a5 ("fs: make sure the timestamps for lazytime inodes eventually get written") Cc: stable@vger.kernel.org Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220227 Signed-off-by: Laveesh Bansal Link: https://patch.msgid.link/20260106145059.543282-2-laveeshb@laveeshbansal.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner [ adapted system_percpu_wq to system_wq for the workqueue used in dirtytime_interval_handler() ] Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

fs: writeback: fix use-after-free in __mark_inode_dirty()

2025-12-06T21:08:24+00:00

[ Upstream commit d02d2c98d25793902f65803ab853b592c7a96b29 ] An use-after-free issue occurred when __mark_inode_dirty() get the bdi_writeback that was in the progress of switching. CPU: 1 PID: 562 Comm: systemd-random- Not tainted 6.6.56-gb4403bd46a8e #1 ...... pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : __mark_inode_dirty+0x124/0x418 lr : __mark_inode_dirty+0x118/0x418 sp : ffffffc08c9dbbc0 ........ Call trace: __mark_inode_dirty+0x124/0x418 generic_update_time+0x4c/0x60 file_modified+0xcc/0xd0 ext4_buffered_write_iter+0x58/0x124 ext4_file_write_iter+0x54/0x704 vfs_write+0x1c0/0x308 ksys_write+0x74/0x10c __arm64_sys_write+0x1c/0x28 invoke_syscall+0x48/0x114 el0_svc_common.constprop.0+0xc0/0xe0 do_el0_svc+0x1c/0x28 el0_svc+0x40/0xe4 el0t_64_sync_handler+0x120/0x12c el0t_64_sync+0x194/0x198 Root cause is: systemd-random-seed kworker ---------------------------------------------------------------------- ___mark_inode_dirty inode_switch_wbs_work_fn spin_lock(&inode->i_lock); inode_attach_wb locked_inode_to_wb_and_lock_list get inode->i_wb spin_unlock(&inode->i_lock); spin_lock(&wb->list_lock) spin_lock(&inode->i_lock) inode_io_list_move_locked spin_unlock(&wb->list_lock) spin_unlock(&inode->i_lock) spin_lock(&old_wb->list_lock) inode_do_switch_wbs spin_lock(&inode->i_lock) inode->i_wb = new_wb spin_unlock(&inode->i_lock) spin_unlock(&old_wb->list_lock) wb_put_many(old_wb, nr_switched) cgwb_release old wb released wb_wakeup_delayed() accesses wb, then trigger the use-after-free issue Fix this race condition by holding inode spinlock until wb_wakeup_delayed() finished. Signed-off-by: Jiufei Xue Link: https://lore.kernel.org/20250728100715.3863241-1-jiufei.xue@samsung.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner Signed-off-by: Nazar Kalashnikov Signed-off-by: Greg Kroah-Hartman

writeback: fix call of incorrect macro

2023-05-17T09:48:10+00:00

[ Upstream commit 3e46c89c74f2c38e5337d2cf44b0b551adff1cb4 ] the variable 'history' is of type u16, it may be an error that the hweight32 macro was used for it I guess macro hweight16 should be used Found by Linux Verification Center (linuxtesting.org) with SVACE. Fixes: 2a81490811d0 ("writeback: implement foreign cgroup inode detection") Signed-off-by: Maxim Korotkov Reviewed-by: Jan Kara Link: https://lore.kernel.org/r/20230119104443.3002-1-korotkov.maxim.s@gmail.com Signed-off-by: Jens Axboe Signed-off-by: Sasha Levin

writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs

2023-05-17T09:47:34+00:00

[ Upstream commit 1ba1199ec5747f475538c0d25a32804e5ba1dfde ] KASAN report null-ptr-deref: ================================================================== BUG: KASAN: null-ptr-deref in bdi_split_work_to_wbs+0x5c5/0x7b0 Write of size 8 at addr 0000000000000000 by task sync/943 CPU: 5 PID: 943 Comm: sync Tainted: 6.3.0-rc5-next-20230406-dirty #461 Call Trace: dump_stack_lvl+0x7f/0xc0 print_report+0x2ba/0x340 kasan_report+0xc4/0x120 kasan_check_range+0x1b7/0x2e0 __kasan_check_write+0x24/0x40 bdi_split_work_to_wbs+0x5c5/0x7b0 sync_inodes_sb+0x195/0x630 sync_inodes_one_sb+0x3a/0x50 iterate_supers+0x106/0x1b0 ksys_sync+0x98/0x160 [...] ================================================================== The race that causes the above issue is as follows: cpu1 cpu2 -------------------------|------------------------- inode_switch_wbs INIT_WORK(&isw->work, inode_switch_wbs_work_fn) queue_rcu_work(isw_wq, &isw->work) // queue_work async inode_switch_wbs_work_fn wb_put_many(old_wb, nr_switched) percpu_ref_put_many ref->data->release(ref) cgwb_release queue_work(cgwb_release_wq, &wb->release_work) // queue_work async &wb->release_work cgwb_release_workfn ksys_sync iterate_supers sync_inodes_one_sb sync_inodes_sb bdi_split_work_to_wbs kmalloc(sizeof(*work), GFP_ATOMIC) // alloc memory failed percpu_ref_exit ref->data = NULL kfree(data) wb_get(wb) percpu_ref_get(&wb->refcnt) percpu_ref_get_many(ref, 1) atomic_long_add(nr, &ref->data->count) atomic64_add(i, v) // trigger null-ptr-deref bdi_split_work_to_wbs() traverses &bdi->wb_list to split work into all wbs. If the allocation of new work fails, the on-stack fallback will be used and the reference count of the current wb is increased afterwards. If cgroup writeback membership switches occur before getting the reference count and the current wb is released as old_wd, then calling wb_get() or wb_put() will trigger the null pointer dereference above. This issue was introduced in v4.3-rc7 (see fix tag1). Both sync_inodes_sb() and __writeback_inodes_sb_nr() calls to bdi_split_work_to_wbs() can trigger this issue. For scenarios called via sync_inodes_sb(), originally commit 7fc5854f8c6e ("writeback: synchronize sync(2) against cgroup writeback membership switches") reduced the possibility of the issue by adding wb_switch_rwsem, but in v5.14-rc1 (see fix tag2) removed the "inode_io_list_del_locked(inode, old_wb)" from inode_switch_wbs_work_fn() so that wb->state contains WB_has_dirty_io, thus old_wb is not skipped when traversing wbs in bdi_split_work_to_wbs(), and the issue becomes easily reproducible again. To solve this problem, percpu_ref_exit() is called under RCU protection to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs(). Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(), and skip the current wb if wb_tryget() fails because the wb has already been shutdown. Link: https://lkml.kernel.org/r/20230410130826.1492525-1-libaokun1@huawei.com Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones") Signed-off-by: Baokun Li Reviewed-by: Jan Kara Acked-by: Tejun Heo Cc: Alexander Viro Cc: Andreas Dilger Cc: Christian Brauner Cc: Dennis Zhou Cc: Hou Tao Cc: yangerkun Cc: Zhang Yi Cc: Jens Axboe Cc: Signed-off-by: Andrew Morton Signed-off-by: Sasha Levin

fs-writeback: writeback_sb_inodes：Recalculate 'wrote' according skipped pages

2022-06-09T08:21:22+00:00

commit 68f4c6eba70df70a720188bce95c85570ddfcc87 upstream. Commit 505a666ee3fc ("writeback: plug writeback in wb_writeback() and writeback_inodes_wb()") has us holding a plug during wb_writeback, which may cause a potential ABBA dead lock: wb_writeback fat_file_fsync blk_start_plug(&plug) for (;;) { iter i-1: some reqs have been added into plug->mq_list // LOCK A iter i: progress = __writeback_inodes_wb(wb, work) . writeback_sb_inodes // fat's bdev . __writeback_single_inode . . generic_writepages . . __block_write_full_page . . . . __generic_file_fsync . . . . sync_inode_metadata . . . . writeback_single_inode . . . . __writeback_single_inode . . . . fat_write_inode . . . . __fat_write_inode . . . . sync_dirty_buffer // fat's bdev . . . . lock_buffer(bh) // LOCK B . . . . submit_bh . . . . blk_mq_get_tag // LOCK A . . . trylock_buffer(bh) // LOCK B . . . redirty_page_for_writepage . . . wbc->pages_skipped++ . . --wbc->nr_to_write . wrote += write_chunk - wbc.nr_to_write // wrote > 0 . requeue_inode . redirty_tail_locked if (progress) // progress > 0 continue; iter i+1: queue_io // similar process with iter i, infinite for-loop ! } blk_finish_plug(&plug) // flush plug won't be called Above process triggers a hungtask like: [ 399.044861] INFO: task bb:2607 blocked for more than 30 seconds. [ 399.046824] Not tainted 5.18.0-rc1-00005-gefae4d9eb6a2-dirty [ 399.051539] task:bb state:D stack: 0 pid: 2607 ppid: 2426 flags:0x00004000 [ 399.051556] Call Trace: [ 399.051570] __schedule+0x480/0x1050 [ 399.051592] schedule+0x92/0x1a0 [ 399.051602] io_schedule+0x22/0x50 [ 399.051613] blk_mq_get_tag+0x1d3/0x3c0 [ 399.051640] __blk_mq_alloc_requests+0x21d/0x3f0 [ 399.051657] blk_mq_submit_bio+0x68d/0xca0 [ 399.051674] __submit_bio+0x1b5/0x2d0 [ 399.051708] submit_bio_noacct+0x34e/0x720 [ 399.051718] submit_bio+0x3b/0x150 [ 399.051725] submit_bh_wbc+0x161/0x230 [ 399.051734] __sync_dirty_buffer+0xd1/0x420 [ 399.051744] sync_dirty_buffer+0x17/0x20 [ 399.051750] __fat_write_inode+0x289/0x310 [ 399.051766] fat_write_inode+0x2a/0xa0 [ 399.051783] __writeback_single_inode+0x53c/0x6f0 [ 399.051795] writeback_single_inode+0x145/0x200 [ 399.051803] sync_inode_metadata+0x45/0x70 [ 399.051856] __generic_file_fsync+0xa3/0x150 [ 399.051880] fat_file_fsync+0x1d/0x80 [ 399.051895] vfs_fsync_range+0x40/0xb0 [ 399.051929] __x64_sys_fsync+0x18/0x30 In my test, 'need_resched()' (which is imported by 590dca3a71 "fs-writeback: unplug before cond_resched in writeback_sb_inodes") in function 'writeback_sb_inodes()' seldom comes true, unless cond_resched() is deleted from write_cache_pages(). Fix it by correcting wrote number according number of skipped pages in writeback_sb_inodes(). Goto Link to find a reproducer. Link: https://bugzilla.kernel.org/show_bug.cgi?id=215837 Cc: stable@vger.kernel.org # v4.3 Signed-off-by: Zhihao Cheng Reviewed-by: Jan Kara Reviewed-by: Christoph Hellwig Link: https://lore.kernel.org/r/20220510133805.1988292-1-chengzhihao1@huawei.com Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman

writeback: fix obtain a reference to a freeing memcg css

2021-07-14T14:56:31+00:00

[ Upstream commit 8b0ed8443ae6458786580d36b7d5f8125535c5d4 ] The caller of wb_get_create() should pin the memcg, because wb_get_create() relies on this guarantee. The rcu read lock only can guarantee that the memcg css returned by css_from_id() cannot be released, but the reference of the memcg can be zero. rcu_read_lock() memcg_css = css_from_id() wb_get_create(memcg_css) cgwb_create(memcg_css) // css_get can change the ref counter from 0 back to 1 css_get(memcg_css) rcu_read_unlock() Fix it by holding a reference to the css before calling wb_get_create(). This is not a problem I encountered in the real world. Just the result of a code review. Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates") Link: https://lore.kernel.org/r/20210402091145.80635-1-songmuchun@bytedance.com Signed-off-by: Muchun Song Acked-by: Michal Hocko Acked-by: Tejun Heo Signed-off-by: Jan Kara Signed-off-by: Sasha Levin

writeback, cgroup: increment isw_nr_in_flight before grabbing an inode

2021-07-14T14:56:02+00:00

[ Upstream commit 8826ee4fe75051f8cbfa5d4a9aa70565938e724c ] isw_nr_in_flight is used to determine whether the inode switch queue should be flushed from the umount path. Currently it's increased after grabbing an inode and even scheduling the switch work. It means the umount path can walk past cleanup_offline_cgwb() with active inode references, which can result in a "Busy inodes after unmount." message and use-after-free issues (with inode->i_sb which gets freed). Fix it by incrementing isw_nr_in_flight before doing anything with the inode and decrementing in the case when switching wasn't scheduled. The problem hasn't yet been seen in the real life and was discovered by Jan Kara by looking into the code. Link: https://lkml.kernel.org/r/20210608230225.2078447-4-guro@fb.com Signed-off-by: Roman Gushchin Suggested-by: Jan Kara Reviewed-by: Jan Kara Cc: Alexander Viro Cc: Dave Chinner Cc: Dennis Zhou Cc: Tejun Heo Cc: Jens Axboe Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin

block_dump: remove block_dump feature in mark_inode_dirty()

2021-07-14T14:55:58+00:00

[ Upstream commit 12e0613715e1cf305fffafaf0e89d810d9a85cc0 ] block_dump is an old debugging interface, one of it's functions is used to print the information about who write which file on disk. If we enable block_dump through /proc/sys/vm/block_dump and turn on debug log level, we can gather information about write process name, target file name and disk from kernel message. This feature is realized in block_dump___mark_inode_dirty(), it print above information into kernel message directly when marking inode dirty, so it is noisy and can easily trigger log storm. At the same time, get the dentry refcount is also not safe, we found it will lead to deadlock on ext4 file system with data=journal mode. After tracepoints has been introduced into the kernel, we got a tracepoint in __mark_inode_dirty(), which is a better replacement of block_dump___mark_inode_dirty(). The only downside is that it only trace the inode number and not a file name, but it probably doesn't matter because the original printed file name in block_dump is not accurate in some cases, and we can still find it through the inode number and device id. So this patch delete the dirting inode part of block_dump feature. Signed-off-by: zhangyi (F) Reviewed-by: Jan Kara Reviewed-by: Christoph Hellwig Link: https://lore.kernel.org/r/20210313030146.2882027-2-yi.zhang@huawei.com Signed-off-by: Jens Axboe Signed-off-by: Sasha Levin

fs: fix lazytime expiration handling in __writeback_single_inode()

2021-01-27T10:54:53+00:00

commit 1e249cb5b7fc09ff216aa5a12f6c302e434e88f9 upstream. When lazytime is enabled and an inode is being written due to its in-memory updated timestamps having expired, either due to a sync() or syncfs() system call or due to dirtytime_expire_interval having elapsed, the VFS needs to inform the filesystem so that the filesystem can copy the inode's timestamps out to the on-disk data structures. This is done by __writeback_single_inode() calling mark_inode_dirty_sync(), which then calls ->dirty_inode(I_DIRTY_SYNC). However, this occurs after __writeback_single_inode() has already cleared the dirty flags from ->i_state. This causes two bugs: - mark_inode_dirty_sync() redirties the inode, causing it to remain dirty. This wastefully causes the inode to be written twice. But more importantly, it breaks cases where sync_filesystem() is expected to clean dirty inodes. This includes the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (as reported at https://lore.kernel.org/r/20200306004555.GB225345@gmail.com), as well as possibly filesystem freezing (freeze_super()). - Since ->i_state doesn't contain I_DIRTY_TIME when ->dirty_inode() is called from __writeback_single_inode() for lazytime expiration, xfs_fs_dirty_inode() ignores the notification. (XFS only cares about lazytime expirations, and it assumes that i_state will contain I_DIRTY_TIME during those.) Therefore, lazy timestamps aren't persisted by sync(), syncfs(), or dirtytime_expire_interval on XFS. Fix this by moving the call to mark_inode_dirty_sync() to earlier in __writeback_single_inode(), before the dirty flags are cleared from i_state. This makes filesystems be properly notified of the timestamp expiration, and it avoids incorrectly redirtying the inode. This fixes xfstest generic/580 (which tests FS_IOC_REMOVE_ENCRYPTION_KEY) when run on ext4 or f2fs with lazytime enabled. It also fixes the new lazytime xfstest I've proposed, which reproduces the above-mentioned XFS bug (https://lore.kernel.org/r/20210105005818.92978-1-ebiggers@kernel.org). Alternatively, we could call ->dirty_inode(I_DIRTY_SYNC) directly. But due to the introduction of I_SYNC_QUEUED, mark_inode_dirty_sync() is the right thing to do because mark_inode_dirty_sync() now knows not to move the inode to a writeback list if it is currently queued for sync. Fixes: 0ae45f63d4ef ("vfs: add support for a lazytime mount option") Cc: stable@vger.kernel.org Depends-on: 5afced3bf281 ("writeback: Avoid skipping inode writeback") Link: https://lore.kernel.org/r/20210112190253.64307-2-ebiggers@kernel.org Suggested-by: Jan Kara Reviewed-by: Christoph Hellwig Reviewed-by: Jan Kara Signed-off-by: Eric Biggers Signed-off-by: Jan Kara Signed-off-by: Greg Kroah-Hartman