[PATCH] btrfs: properly enable async discard when switching from RO->RW

[Chris Mason <clm@fb.com>]

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(+)

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:
 	/*
-- 
2.34.1