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

["Darrick J. Wong" <darrick.wong@oracle.com>]

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?

--D

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