Re: [PATCH] mm, swap: unlock inode in error path of claim_swapfile

[Naohiro Aota <naohiro.aota@wdc.com>]

On Tue, Feb 04, 2020 at 07:42:29AM -0800, Darrick J. Wong wrote:
>On Tue, Feb 04, 2020 at 06:59:43PM +0900, Naohiro Aota 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 unlocking the inode on the error path. It
>> also reverts blocksize and releases bdev, so that the caller can safely
>> forget about the inode.
>>
>>     =====================================
>>     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
>>
>> Fixes: 1638045c3677 ("mm: set S_SWAPFILE on blockdev swap devices")
>> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>> ---
>>  mm/swapfile.c | 29 ++++++++++++++++++++++-------
>>  1 file changed, 22 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index bb3261d45b6a..dd5d7fa42282 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -2886,24 +2886,37 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
>>  		p->old_block_size = block_size(p->bdev);
>>  		error = set_blocksize(p->bdev, PAGE_SIZE);
>>  		if (error < 0)
>> -			return error;
>> +			goto err;
>>  		/*
>>  		 * Zoned block devices contain zones that have a sequential
>>  		 * write only restriction.  Hence zoned block devices are not
>>  		 * suitable for swapping.  Disallow them here.
>>  		 */
>> -		if (blk_queue_is_zoned(p->bdev->bd_queue))
>> -			return -EINVAL;
>> +		if (blk_queue_is_zoned(p->bdev->bd_queue)) {
>> +			error = -EINVAL;
>> +			goto err;
>> +		}
>>  		p->flags |= SWP_BLKDEV;
>>  	} else if (S_ISREG(inode->i_mode)) {
>>  		p->bdev = inode->i_sb->s_bdev;
>>  	}
>>
>>  	inode_lock(inode);
>> -	if (IS_SWAPFILE(inode))
>> -		return -EBUSY;
>> +	if (IS_SWAPFILE(inode)) {
>> +		inode_unlock(inode);
>> +		error = -EBUSY;
>> +		goto err;
>> +	}
>>
>>  	return 0;
>> +
>> +err:
>> +	if (S_ISBLK(inode->i_mode)) {
>> +		set_blocksize(p->bdev, p->old_block_size);
>> +		blkdev_put(p->bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
>> +	}
>> +
>> +	return error;
>>  }
>>
>>
>> @@ -3157,10 +3170,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>  	mapping = swap_file->f_mapping;
>>  	inode = mapping->host;
>>
>> -	/* If S_ISREG(inode->i_mode) will do inode_lock(inode); */
>> +	/* do inode_lock(inode); */
>
>What if we made this function responsible for calling inode_lock (and
>unlock) instead of splitting it between sys_swapon and claim_swapfile?

I think we cannot take inode_lock before claim_swapfile() because we can
have circular locking dependency as:

claim_swapfile()
-> blkdev_get() 
    -> __blkdev_get()
       -> mutex_lock(&bdev->bd_mutex)
       -> bd_set_size()
          -> inode_lock(&bdev->bd_inode);

So, one thing we can do is to move inode_lock() and "if (IS_SWAPFILE(..))
..." out of claim_swapfile(). In this case, the "bad_swap" section must
check if "inode_is_locked" to call "inode_unlock".

>
>--D
>
>>  	error = claim_swapfile(p, inode);
>> -	if (unlikely(error))
>> +	if (unlikely(error)) {
>> +		inode = NULL;
>>  		goto bad_swap;
>> +	}
>>
>>  	/*
>>  	 * Read the swap header.
>> --
>> 2.25.0
>>