Re: [PATCH v2] mm, swap: move inode_lock out of claim_swapfile

Andrew Morton
2020-02-10T04:16:15Z

On Thu, 6 Feb 2020 18:01:32 +0900 Naohiro Aota <naohiro.aota@wdc.com> wrote:

> claim_swapfile() currently keeps the inode locked when it is successful, or
> the file is already swapfile (with -EBUSY). And, on the other error cases,
> it does not lock the inode.
>
> This inconsistency of the lock state and return value is quite confusing
> and actually causing a bad unlock balance as below in the "bad_swap"
> section of __do_sys_swapon().
>
> This commit fixes this issue by moving the inode_lock() and IS_SWAPFILE
> check out of claim_swapfile(). The inode is unlocked in
> "bad_swap_unlock_inode" section, so that the inode is ensured to be
> unlocked at "bad_swap". Thus, error handling codes after the locking now
> jumps to "bad_swap_unlock_inode" instead of "bad_swap".
>
> =====================================
> WARNING: bad unlock balance detected!
> 5.5.0-rc7+ #176 Not tainted
> -------------------------------------
> swapon/4294 is trying to release lock (&sb->s_type->i_mutex_key) at:
> [<ffffffff8173a6eb>] __do_sys_swapon+0x94b/0x3550
> but there are no more locks to release!
>
> other info that might help us debug this:
> no locks held by swapon/4294.
>
> stack backtrace:
> CPU: 5 PID: 4294 Comm: swapon Not tainted 5.5.0-rc7-BTRFS-ZNS+ #176
> Hardware name: ASUS All Series/H87-PRO, BIOS 2102 07/29/2014
> Call Trace:
> dump_stack+0xa1/0xea
> ? __do_sys_swapon+0x94b/0x3550
> print_unlock_imbalance_bug.cold+0x114/0x123
> ? __do_sys_swapon+0x94b/0x3550
> lock_release+0x562/0xed0
> ? kvfree+0x31/0x40
> ? lock_downgrade+0x770/0x770
> ? kvfree+0x31/0x40
> ? rcu_read_lock_sched_held+0xa1/0xd0
> ? rcu_read_lock_bh_held+0xb0/0xb0
> up_write+0x2d/0x490
> ? kfree+0x293/0x2f0
> __do_sys_swapon+0x94b/0x3550
> ? putname+0xb0/0xf0
> ? kmem_cache_free+0x2e7/0x370
> ? do_sys_open+0x184/0x3e0
> ? generic_max_swapfile_size+0x40/0x40
> ? do_syscall_64+0x27/0x4b0
> ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
> ? lockdep_hardirqs_on+0x38c/0x590
> __x64_sys_swapon+0x54/0x80
> do_syscall_64+0xa4/0x4b0
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x7f15da0a0dc7
>
> mm/swapfile.c | 41 ++++++++++++++++++++---------------------
> 1 file changed, 20 insertions(+), 21 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index bb3261d45b6a..2c4c349e1101 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c

Look correct to me.

But I don't think this code at the end of sys_swapon():

if (inode)
inode_unlock(inode);

will ever execute? `inode' is always NULL here?