From: Chris Mason clm@fb.com
commit 981a37bab5e5f16137266d3f00cf2bd018af36ef upstream.
The async discard uses the BTRFS_FS_DISCARD_RUNNING bit in the fs_info to force discards off when the filesystem has aborted or we're generally not able to run discards. This gets flipped on when we're mounted rw, and also when we go from ro->rw.
Commit 63a7cb13071842 ("btrfs: auto enable discard=async when possible") enabled async discard by default, and this meant "mount -o ro /dev/xxx /yyy" had async discards turned on.
Unfortunately, this meant our check in btrfs_remount_cleanup() would see that discards are already on:
/* If we toggled discard async */ if (!btrfs_raw_test_opt(old_opts, DISCARD_ASYNC) && btrfs_test_opt(fs_info, DISCARD_ASYNC)) btrfs_discard_resume(fs_info);
So, we'd never call btrfs_discard_resume() when remounting the root filesystem from ro->rw.
drgn shows this really nicely:
import os import sys
from drgn.helpers.linux.fs import path_lookup from drgn import NULL, Object, Type, cast
def btrfs_sb(sb): return cast("struct btrfs_fs_info *", sb.s_fs_info)
if len(sys.argv) == 1: path = "/" else: path = sys.argv[1]
fs_info = cast("struct btrfs_fs_info *", path_lookup(prog, path).mnt.mnt_sb.s_fs_info)
BTRFS_FS_DISCARD_RUNNING = 1 << prog['BTRFS_FS_DISCARD_RUNNING'] if fs_info.flags & BTRFS_FS_DISCARD_RUNNING: print("discard running flag is on") else: print("discard running flag is off")
[root]# mount | grep nvme /dev/nvme0n1p3 on / type btrfs (rw,relatime,compress-force=zstd:3,ssd,discard=async,space_cache=v2,subvolid=5,subvol=/)
[root]# ./discard_running.drgn discard running flag is off
[root]# mount -o remount,discard=sync / [root]# mount -o remount,discard=async / [root]# ./discard_running.drgn discard running flag is on
The fix is to call btrfs_discard_resume() when we're going from ro->rw. It already checks to make sure the async discard flag is on, so it'll do the right thing.
Fixes: 63a7cb13071842 ("btrfs: auto enable discard=async when possible") CC: stable@vger.kernel.org # 6.3+ Reviewed-by: Boris Burkov boris@bur.io Signed-off-by: Chris Mason clm@fb.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/super.c | 6 ++++++ 1 file changed, 6 insertions(+)
--- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -1840,6 +1840,12 @@ static int btrfs_remount(struct super_bl btrfs_clear_sb_rdonly(sb);
set_bit(BTRFS_FS_OPEN, &fs_info->flags); + + /* + * If we've gone from readonly -> read/write, we need to get + * our sync/async discard lists in the right state. + */ + btrfs_discard_resume(fs_info); } out: /*