kernel/git/stable/linux.git/fs/btrfs/discard.c, branch master

btrfs: handle discarding fully-remapped block groups

2026-02-03T06:54:36+00:00

Discard normally works by iterating over the free-space entries of a block group. This doesn't work for fully-remapped block groups, as we removed their free-space entries when we started relocation. For sync discard, call btrfs_discard_extent() when we commit the transaction in which the last identity remap was removed. For async discard, add a new function btrfs_trim_fully_remapped_block_group() to be called by the discard worker, which iterates over the block group's range using the normal async discard rules. Once we reach the end, remove the chunk's stripes and device extents to get back its free space. Reviewed-by: Boris Burkov Signed-off-by: Mark Harmstone Signed-off-by: David Sterba

btrfs: add extended version of struct block_group_item

2026-02-03T06:54:34+00:00

Add a struct btrfs_block_group_item_v2, which is used in the block group tree if the remap-tree incompat flag is set. This adds two new fields to the block group item: `remap_bytes` and `identity_remap_count`. `remap_bytes` records the amount of data that's physically within this block group, but nominally in another, remapped block group. This is necessary because this data will need to be moved first if this block group is itself relocated. If `remap_bytes` > 0, this is an indicator to the relocation thread that it will need to search the remap-tree for backrefs. A block group must also have `remap_bytes` == 0 before it can be dropped. `identity_remap_count` records how many identity remap items are located in the remap tree for this block group. When relocation is begun for this block group, this is set to the number of holes in the free-space tree for this range. As identity remaps are converted into actual remaps by the relocation process, this number is decreased. Once it reaches 0, either because of relocation or because extents have been deleted, the block group has been fully remapped and its chunk's device extents are removed. Reviewed-by: Boris Burkov Signed-off-by: Mark Harmstone Signed-off-by: David Sterba

btrfs: use verbose assert at peek_discard_list()

2025-05-15T12:30:55+00:00

We now have a verbose variant of ASSERT() so that we can print the value of the block group's discard_index. So use it for better problem analysis in case the assertion is triggered. Reviewed-by: Boris Burkov Reviewed-by: Daniel Vacek Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: fix discard worker infinite loop after disabling discard

2025-05-12T19:38:56+00:00

If the discard worker is running and there's currently only one block group, that block group is a data block group, it's in the unused block groups discard list and is being used (it got an extent allocated from it after becoming unused), the worker can end up in an infinite loop if a transaction abort happens or the async discard is disabled (during remount or unmount for example). This happens like this: 1) Task A, the discard worker, is at peek_discard_list() and find_next_block_group() returns block group X; 2) Block group X is in the unused block groups discard list (its discard index is BTRFS_DISCARD_INDEX_UNUSED) since at some point in the past it become an unused block group and was added to that list, but then later it got an extent allocated from it, so its ->used counter is not zero anymore; 3) The current transaction is aborted by task B and we end up at __btrfs_handle_fs_error() in the transaction abort path, where we call btrfs_discard_stop(), which clears BTRFS_FS_DISCARD_RUNNING from fs_info, and then at __btrfs_handle_fs_error() we set the fs to RO mode (setting SB_RDONLY in the super block's s_flags field); 4) Task A calls __add_to_discard_list() with the goal of moving the block group from the unused block groups discard list into another discard list, but at __add_to_discard_list() we end up doing nothing because btrfs_run_discard_work() returns false, since the super block has SB_RDONLY set in its flags and BTRFS_FS_DISCARD_RUNNING is not set anymore in fs_info->flags. So block group X remains in the unused block groups discard list; 5) Task A then does a goto into the 'again' label, calls find_next_block_group() again we gets block group X again. Then it repeats the previous steps over and over since there are not other block groups in the discard lists and block group X is never moved out of the unused block groups discard list since btrfs_run_discard_work() keeps returning false and therefore __add_to_discard_list() doesn't move block group X out of that discard list. When this happens we can get a soft lockup report like this: [71.957] watchdog: BUG: soft lockup - CPU#0 stuck for 27s! [kworker/u4:3:97] [71.957] Modules linked in: xfs af_packet rfkill (...) [71.957] CPU: 0 UID: 0 PID: 97 Comm: kworker/u4:3 Tainted: G W 6.14.2-1-default #1 openSUSE Tumbleweed 968795ef2b1407352128b466fe887416c33af6fa [71.957] Tainted: [W]=WARN [71.957] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014 [71.957] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs] [71.957] RIP: 0010:btrfs_discard_workfn+0xc4/0x400 [btrfs] [71.957] Code: c1 01 48 83 (...) [71.957] RSP: 0018:ffffafaec03efe08 EFLAGS: 00000246 [71.957] RAX: ffff897045500000 RBX: ffff8970413ed8d0 RCX: 0000000000000000 [71.957] RDX: 0000000000000001 RSI: ffff8970413ed8d0 RDI: 0000000a8f1272ad [71.957] RBP: 0000000a9d61c60e R08: ffff897045500140 R09: 8080808080808080 [71.957] R10: ffff897040276800 R11: fefefefefefefeff R12: ffff8970413ed860 [71.957] R13: ffff897045500000 R14: ffff8970413ed868 R15: 0000000000000000 [71.957] FS: 0000000000000000(0000) GS:ffff89707bc00000(0000) knlGS:0000000000000000 [71.957] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [71.957] CR2: 00005605bcc8d2f0 CR3: 000000010376a001 CR4: 0000000000770ef0 [71.957] PKRU: 55555554 [71.957] Call Trace: [71.957] [71.957] process_one_work+0x17e/0x330 [71.957] worker_thread+0x2ce/0x3f0 [71.957] ? __pfx_worker_thread+0x10/0x10 [71.957] kthread+0xef/0x220 [71.957] ? __pfx_kthread+0x10/0x10 [71.957] ret_from_fork+0x34/0x50 [71.957] ? __pfx_kthread+0x10/0x10 [71.957] ret_from_fork_asm+0x1a/0x30 [71.957] [71.957] Kernel panic - not syncing: softlockup: hung tasks [71.987] CPU: 0 UID: 0 PID: 97 Comm: kworker/u4:3 Tainted: G W L 6.14.2-1-default #1 openSUSE Tumbleweed 968795ef2b1407352128b466fe887416c33af6fa [71.989] Tainted: [W]=WARN, [L]=SOFTLOCKUP [71.989] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014 [71.991] Workqueue: btrfs_discard btrfs_discard_workfn [btrfs] [71.992] Call Trace: [71.993] [71.994] dump_stack_lvl+0x5a/0x80 [71.994] panic+0x10b/0x2da [71.995] watchdog_timer_fn.cold+0x9a/0xa1 [71.996] ? __pfx_watchdog_timer_fn+0x10/0x10 [71.997] __hrtimer_run_queues+0x132/0x2a0 [71.997] hrtimer_interrupt+0xff/0x230 [71.998] __sysvec_apic_timer_interrupt+0x55/0x100 [71.999] sysvec_apic_timer_interrupt+0x6c/0x90 [72.000] [72.000] [72.001] asm_sysvec_apic_timer_interrupt+0x1a/0x20 [72.002] RIP: 0010:btrfs_discard_workfn+0xc4/0x400 [btrfs] [72.002] Code: c1 01 48 83 (...) [72.005] RSP: 0018:ffffafaec03efe08 EFLAGS: 00000246 [72.006] RAX: ffff897045500000 RBX: ffff8970413ed8d0 RCX: 0000000000000000 [72.006] RDX: 0000000000000001 RSI: ffff8970413ed8d0 RDI: 0000000a8f1272ad [72.007] RBP: 0000000a9d61c60e R08: ffff897045500140 R09: 8080808080808080 [72.008] R10: ffff897040276800 R11: fefefefefefefeff R12: ffff8970413ed860 [72.009] R13: ffff897045500000 R14: ffff8970413ed868 R15: 0000000000000000 [72.010] ? btrfs_discard_workfn+0x51/0x400 [btrfs 23b01089228eb964071fb7ca156eee8cd3bf996f] [72.011] process_one_work+0x17e/0x330 [72.012] worker_thread+0x2ce/0x3f0 [72.013] ? __pfx_worker_thread+0x10/0x10 [72.014] kthread+0xef/0x220 [72.014] ? __pfx_kthread+0x10/0x10 [72.015] ret_from_fork+0x34/0x50 [72.015] ? __pfx_kthread+0x10/0x10 [72.016] ret_from_fork_asm+0x1a/0x30 [72.017] [72.017] Kernel Offset: 0x15000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff) [72.019] Rebooting in 90 seconds.. So fix this by making sure we move a block group out of the unused block groups discard list when calling __add_to_discard_list(). Fixes: 2bee7eb8bb81 ("btrfs: discard one region at a time in async discard") Link: https://bugzilla.suse.com/show_bug.cgi?id=1242012 CC: stable@vger.kernel.org # 5.10+ Reviewed-by: Boris Burkov Reviewed-by: Daniel Vacek Signed-off-by: Filipe Manana Signed-off-by: David Sterba

btrfs: make btrfs_discard_workfn() block_group ref explicit

2025-03-18T19:35:51+00:00

Currently, the async discard machinery owns a ref to the block_group when the block_group is queued on a discard list. However, to handle races with discard cancellation and the discard workfn, we have a specific logic to detect that the block_group is *currently* running in the workfn, to protect the workfn's usage amidst cancellation. As far as I can tell, this doesn't have any overt bugs (though finish_discard_pass() and remove_from_discard_list() racing can have a surprising outcome for the caller of remove_from_discard_list() in that it is again added at the end). But it is needlessly complicated to rely on locking and the nullity of discard_ctl->block_group. Simplify this significantly by just taking a refcount while we are in the workfn and unconditionally drop it in both the remove and workfn paths, regardless of if they race. Reviewed-by: Filipe Manana Signed-off-by: Boris Burkov Signed-off-by: David Sterba

btrfs: constify more pointer parameters

2024-09-10T14:51:22+00:00

Continue adding const to parameters. This is for clarity and minor addition to safety. There are some minor effects, in the assembly code and .ko measured on release config. Signed-off-by: David Sterba

btrfs: unexport btrfs_run_discard_work and make it static

2023-06-19T11:59:25+00:00

Mark btrfs_run_discard_work static and move it above its callers. Signed-off-by: Johannes Thumshirn Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: reinterpret async discard iops_limit=0 as no delay

2023-04-20T22:28:23+00:00

Currently, a limit of 0 results in a hard coded metering over 6 hours. Since the default is a set limit, I suspect no one truly depends on this rather arbitrary setting. Repurpose it for an arguably more useful "unlimited" mode, where the delay is 0. Note that if block groups are too new, or go fully empty, there is still a delay associated with those conditions. Those delays implement heuristics for not trimming a region we are relatively likely to fully overwrite soon. CC: stable@vger.kernel.org # 6.2+ Reviewed-by: Neal Gompa Signed-off-by: Boris Burkov Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: set default discard iops_limit to 1000

2023-04-20T22:28:20+00:00

Previously, the default was a relatively conservative 10. This results in a 100ms delay, so with ~300 discards in a commit, it takes the full 30s till the next commit to finish the discards. On a workstation, this results in the disk never going idle, wasting power/battery, etc. Set the default to 1000, which results in using the smallest possible delay, currently, which is 1ms. This has shown to not pathologically keep the disk busy by the original reporter. Link: https://lore.kernel.org/linux-btrfs/Y%2F+n1wS%2F4XAH7X1p@nz/ Link: https://bugzilla.redhat.com/show_bug.cgi?id=2182228 CC: stable@vger.kernel.org # 6.2+ Reviewed-by: Neal Gompa Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: hold block group refcount during async discard

2023-02-15T18:38:50+00:00

Async discard does not acquire the block group reference count while it holds a reference on the discard list. This is generally OK, as the paths which destroy block groups tend to try to synchronize on cancelling async discard work. However, relying on cancelling work requires careful analysis to be sure it is safe from races with unpinning scheduling more work. While I am unable to find a race with unpinning in the current code for either the unused bgs or relocation paths, I believe we have one in an older version of auto relocation in a Meta internal build. This suggests that this is in fact an error prone model, and could be fragile to future changes to these bg deletion paths. To make this ownership more clear, add a refcount for async discard. If work is queued for a block group, its refcount should be incremented, and when work is completed or canceled, it should be decremented. CC: stable@vger.kernel.org # 5.15+ Signed-off-by: Boris Burkov Signed-off-by: David Sterba