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

From: Naohiro Aota <naohiro.aota@wdc.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	"Darrick J . Wong" <darrick.wong@oracle.com>
Subject: Re: [PATCH v2] mm, swap: move inode_lock out of claim_swapfile
Date: Mon, 10 Feb 2020 13:25:14 +0900	[thread overview]
Message-ID: <20200210042514.h3hhuqs2v3qketjy@naota.dhcp.fujisawa.hgst.com> (raw)
In-Reply-To: <20200209201612.e5f234b357823df574104cb9@linux-foundation.org>

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.

>