| Message ID | 20230605190315.2121377-1-clm@fb.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | btrfs: properly enable async discard when switching from RO->RW | expand |
On Mon, Jun 05, 2023 at 12:03:15PM -0700, Chris Mason wrote: > 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 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 used here is calling 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") > > Reviewed-by: Boris Burkov <boris@bur.io> > Signed-off-by: Chris Mason <clm@fb.com> Added to misc-next, thanks.
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index ec18e2210602..1e212f964224 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -1841,6 +1841,12 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data) 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: /*
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 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 used here is calling 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") Reviewed-by: Boris Burkov <boris@bur.io> Signed-off-by: Chris Mason <clm@fb.com> --- fs/btrfs/super.c | 6 ++++++ 1 file changed, 6 insertions(+)