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@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?
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
