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@xxxxxxx>
> ---
> 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
>
