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

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

On Wed, Feb 05, 2020 at 08:49:16AM +0900, Naohiro Aota wrote:
> 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);

Ah, good point. Thank you for doing the research on that. :)

> 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".

I think I wouldn't rely on inode_is_locked and structure the error
escape as follows:

	err = claim_swapfile()
	if (err)
		goto bad_swap;

	inode_lock()
	if (IS_SWAPFILE)
		goto unlock_swap;

	other_stuff()

unlock_swap:
	inode_unlock()
bad_swap:
	fput()

since that's how we (well, XFS anyway :)) tend to do it.

--D

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