CVE-2022-48920: In the Linux kernel, the following vulnerability has been resolved:  btrfs: get rid of warning on transaction commit when using flushoncommit  When using the flushoncommit mount option, during almost every transaction commit we trigger a warning from __writeback_inodes_sb_nr():    $ cat fs/fs-writeback.c:   (...)   static void __writeback_inodes_sb_nr(struct super_block *sb, ...   {         (...)         WARN_ON(!rwsem_is_locked(&sb->s_umount));         (...)   }   (...)  The trace produced in dmesg looks like the following:    [947.473890] WARNING: CPU: 5 PID: 930 at fs/fs-writeback.c:2610 __writeback_inodes_sb_nr+0x7e/0xb3   [947.481623] Modules linked in: nfsd nls_cp437 cifs asn1_decoder cifs_arc4 fscache cifs_md4 ipmi_ssif   [947.489571] CPU: 5 PID: 930 Comm: btrfs-transacti Not tainted 95.16.3-srb-asrock-00001-g36437ad63879 #186   [947.497969] RIP: 0010:__writeback_inodes_sb_nr+0x7e/0xb3   [947.502097] Code: 24 10 4c 89 44 24 18 c6 (...)   [947.519760] RSP: 0018:ffffc90000777e10 EFLAGS: 00010246   [947.523818] RAX: 0000000000000000 RBX: 0000000000963300 RCX: 0000000000000000   [947.529765] RDX: 0000000000000000 RSI: 000000000000fa51 RDI: ffffc90000777e50   [947.535740] RBP: ffff888101628a90 R08: ffff888100955800 R09: ffff888100956000   [947.541701] R10: 0000000000000002 R11: 0000000000000001 R12: ffff888100963488   [947.547645] R13: ffff888100963000 R14: ffff888112fb7200 R15: ffff888100963460   [947.553621] FS:  0000000000000000(0000) GS:ffff88841fd40000(0000) knlGS:0000000000000000   [947.560537] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033   [947.565122] CR2: 0000000008be50c4 CR3: 000000000220c000 CR4: 00000000001006e0   [947.571072] Call Trace:   [947.572354]  <TASK>   [947.573266]  btrfs_commit_transaction+0x1f1/0x998   [947.576785]  ? start_transaction+0x3ab/0x44e   [947.579867]  ? schedule_timeout+0x8a/0xdd   [947.582716]  transaction_kthread+0xe9/0x156   [947.585721]  ? btrfs_cleanup_transaction.isra.0+0x407/0x407   [947.590104]  kthread+0x131/0x139   [947.592168]  ? set_kthread_struct+0x32/0x32   [947.595174]  ret_from_fork+0x22/0x30   [947.597561]  </TASK>   [947.598553] ---[ end trace 644721052755541c ]---  This is because we started using writeback_inodes_sb() to flush delalloc when committing a transaction (when using -o flushoncommit), in order to avoid deadlocks with filesystem freeze operations. This change was made by commit ce8ea7cc6eb313 ("btrfs: don't call btrfs_start_delalloc_roots in flushoncommit"). After that change we started producing that warning, and every now and then a user reports this since the warning happens too often, it spams dmesg/syslog, and a user is unsure if this reflects any problem that might compromise the filesystem's reliability.  We can not just lock the sb->s_umount semaphore before calling writeback_inodes_sb(), because that would at least deadlock with filesystem freezing, since at fs/super.c:freeze_super() sync_filesystem() is called while we are holding that semaphore in write mode, and that can trigger a transaction commit, resulting in a deadlock. It would also trigger the same type of deadlock in the unmount path. Possibly, it could also introduce some other locking dependencies that lockdep would report.  To fix this call try_to_writeback_inodes_sb() instead of writeback_inodes_sb(), because that will try to read lock sb->s_umount and then will only call writeback_inodes_sb() if it was able to lock it. This is fine because the cases where it can't read lock sb->s_umount are during a filesystem unmount or during a filesystem freeze - in those cases sb->s_umount is write locked and sync_filesystem() is called, which calls writeback_inodes_sb(). In other words, in all cases where we can't take a read lock on sb->s_umount, writeback is already being triggered elsewhere.  An alternative would be to call btrfs_start_delalloc_roots() with a number of pages different from LONG_MAX, for example matching the number of delalloc bytes we currently have, in  ---truncated---