Re: [PATCH v2] mm, swap: move inode_lock out of claim_swapfile

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On Sun, Feb 09, 2020 at 08:16:12PM -0800, Andrew Morton wrote:
On Thu,  6 Feb 2020 18:01:32 +0900 Naohiro Aota <naohiro.aota@xxxxxxx> 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 moving the inode_lock() and IS_SWAPFILE
check out of claim_swapfile(). The inode is unlocked in
"bad_swap_unlock_inode" section, so that the inode is ensured to be
unlocked at "bad_swap". Thus, error handling codes after the locking now
jumps to "bad_swap_unlock_inode" instead of "bad_swap".

    =====================================
    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

 mm/swapfile.c | 41 ++++++++++++++++++++---------------------
 1 file changed, 20 insertions(+), 21 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index bb3261d45b6a..2c4c349e1101 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c

Look correct to me.

But I don't think this code at the end of sys_swapon():

	if (inode)
		inode_unlock(inode);

will ever execute?  `inode' is always NULL here?

On the successful case, inode is not NULL and unlocked here.






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