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

Naohiro Aota
2020-02-10T04:25:19Z

On Sun, Feb 09, 2020 at 08:16:12PM -0800, Andrew Morton wrote:
>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?

On the successful case, inode is not NULL and unlocked here.

>