Re: [RFC PATCH] nfs: avoid swap-over-NFS deadlock

[Jerome Marchand]

On 08/20/2015 02:23 PM, Mel Gorman wrote:
> On Mon, Jul 27, 2015 at 01:25:47PM +0200, Jerome Marchand wrote:
>> On 07/27/2015 12:52 PM, Mel Gorman wrote:
>>> On Wed, Jul 22, 2015 at 03:46:16PM +0200, Jerome Marchand wrote:
>>>> On 07/22/2015 02:23 PM, Trond Myklebust wrote:
>>>>> On Wed, Jul 22, 2015 at 4:10 AM, Jerome Marchand <jmarchan@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> Lockdep warns about a inconsistent {RECLAIM_FS-ON-W} ->
>>>>>> {IN-RECLAIM_FS-W} usage. The culpritt is the inode->i_mutex taken in
>>>>>> nfs_file_direct_write(). This code was introduced by commit a9ab5e840669
>>>>>> ("nfs: page cache invalidation for dio").
>>>>>> This naive test patch avoid to take the mutex on a swapfile and makes
>>>>>> lockdep happy again. However I don't know much about NFS code and I
>>>>>> assume it's probably not the proper solution. Any thought?
>>>>>>
>>>>>> Signed-off-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
>>>>>
>>>>> NFS is not the only O_DIRECT implementation to set the inode->i_mutex.
>>>>> Why can't this be fixed in the generic swap code instead of adding
>>>>> yet-another-exception-for-IS_SWAPFILE?
>>>>
>>>> I meant to cc Mel. Just added him.
>>>>
>>>
>>> Can the full lockdep warning be included as it'll be easier to see then if
>>> the generic swap code can somehow special case this? Currently, generic
>>> swapping does not not need to care about how the filesystem locked.
>>> For most filesystems, it's writing directly to the blocks on disk and
>>> bypassing the FS. In the NFS case it'd be surprising to find that there
>>> also are dirty pages in page cache that belong to the swap file as it's
>>> going to cause corruption. If there is any special casing it would to only
>>> attempt the invalidation in the !swap case and warn if mapping->nrpages. It
>>> still would look a bit weird but safer than just not acquiring the mutex
>>> and then potentially attempting an invalidation.
>>>
>>
>> [ 6819.501009] =================================
>> [ 6819.501009] [ INFO: inconsistent lock state ]
>> [ 6819.501009] 4.2.0-rc1-shmacct-babka-v2-next-20150709+ #255 Not tainted
>> [ 6819.501009] ---------------------------------
> 
> Thanks. Sorry for the long delay but I finally got back to the bug this
> week. NFS can be modified to special case the swapfile but I was not happy
> with the result for multiple reasons. It took me a while to see a way for
> the core VM to deal with it. What do you think of the following
> approach?

Seems sound to me.

> More importantly, does it work for you?

Yes.

> 
> ---8<---
> nfs: Use swap_lock to prevent parallel swapon activations
> 
> Jerome Marchand reported a lockdep warning as follows
> 
>     [ 6819.501009] =================================
>     [ 6819.501009] [ INFO: inconsistent lock state ]
>     [ 6819.501009] 4.2.0-rc1-shmacct-babka-v2-next-20150709+ #255 Not tainted
>     [ 6819.501009] ---------------------------------
>     [ 6819.501009] inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage.
>     [ 6819.501009] kswapd0/38 [HC0[0]:SC0[0]:HE1:SE1] takes:
>     [ 6819.501009]  (&sb->s_type->i_mutex_key#17){+.+.?.}, at: [<ffffffffa03772a5>] nfs_file_direct_write+0x85/0x3f0 [nfs]
>     [ 6819.501009] {RECLAIM_FS-ON-W} state was registered at:
>     [ 6819.501009]   [<ffffffff81107f51>] mark_held_locks+0x71/0x90
>     [ 6819.501009]   [<ffffffff8110b775>] lockdep_trace_alloc+0x75/0xe0
>     [ 6819.501009]   [<ffffffff81245529>] kmem_cache_alloc_node_trace+0x39/0x440
>     [ 6819.501009]   [<ffffffff81225b8f>] __get_vm_area_node+0x7f/0x160
>     [ 6819.501009]   [<ffffffff81226eb2>] __vmalloc_node_range+0x72/0x2c0
>     [ 6819.501009]   [<ffffffff81227424>] vzalloc+0x54/0x60
>     [ 6819.501009]   [<ffffffff8122c7c8>] SyS_swapon+0x628/0xfc0
>     [ 6819.501009]   [<ffffffff81867772>] entry_SYSCALL_64_fastpath+0x12/0x76
> 
> It's due to NFS acquiring i_mutex since a9ab5e840669 ("nfs: page
> cache invalidation for dio") to invalidate page cache before direct I/O.
> Filesystems may safely acquire i_mutex during direct writes but NFS is unique
> in its treatment of swap files. Ordinarily swap files are supported by the
> core VM looking up the physical block for a given offset in advance. There
> is no physical block for NFS and the direct write paths are used after
> calling mapping->swap_activate.
> 
> The lockdep warning is triggered by swapon(), which is not in reclaim
> context, acquiring the i_mutex to ensure a swapfile is not activated twice.
> 
> swapon does not need the i_mutex for this purpose.  There is a requirement
> that fallocate not be used on swapfiles but this is protected by the inode
> flag S_SWAPFILE and nothing to do with i_mutex. In fact, the current
> protection does nothing for block devices. This patch expands the role
> of swap_lock to protect against parallel activations of block devices and
> swapfiles and removes the use of i_mutex. This both improves the protection
> for swapon and avoids the lockdep warning.
> 
> Reported-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Tested-by: Jerome Marchand <jmarchan@xxxxxxxxxx>

Thanks,
Jerome

> ---
>  mm/swapfile.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 41e4581af7c5..d58ed6833fa3 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1928,9 +1928,9 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		set_blocksize(bdev, old_block_size);
>  		blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
>  	} else {
> -		mutex_lock(&inode->i_mutex);
> +		spin_lock(&swap_lock);
>  		inode->i_flags &= ~S_SWAPFILE;
> -		mutex_unlock(&inode->i_mutex);
> +		spin_unlock(&swap_lock);
>  	}
>  	filp_close(swap_file, NULL);
>  
> @@ -2156,7 +2156,6 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
>  		p->flags |= SWP_BLKDEV;
>  	} else if (S_ISREG(inode->i_mode)) {
>  		p->bdev = inode->i_sb->s_bdev;
> -		mutex_lock(&inode->i_mutex);
>  		if (IS_SWAPFILE(inode))
>  			return -EBUSY;
>  	} else
> @@ -2386,6 +2385,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		goto bad_swap;
>  	}
>  
> +	/* prevent parallel swapons */
> +	spin_lock(&swap_lock);
>  	p->swap_file = swap_file;
>  	mapping = swap_file->f_mapping;
>  
> @@ -2396,13 +2397,14 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  			continue;
>  		if (mapping == q->swap_file->f_mapping) {
>  			error = -EBUSY;
> +			spin_unlock(&swap_lock);
>  			goto bad_swap;
>  		}
>  	}
>  
>  	inode = mapping->host;
> -	/* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
>  	error = claim_swapfile(p, inode);
> +	spin_unlock(&swap_lock);
>  	if (unlikely(error))
>  		goto bad_swap;
>  
> @@ -2543,10 +2545,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  	vfree(swap_map);
>  	vfree(cluster_info);
>  	if (swap_file) {
> -		if (inode && S_ISREG(inode->i_mode)) {
> -			mutex_unlock(&inode->i_mutex);
> +		if (inode && S_ISREG(inode->i_mode))
>  			inode = NULL;
> -		}
>  		filp_close(swap_file, NULL);
>  	}
>  out:
> @@ -2556,8 +2556,6 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  	}
>  	if (name)
>  		putname(name);
> -	if (inode && S_ISREG(inode->i_mode))
> -		mutex_unlock(&inode->i_mutex);
>  	return error;
>  }
>  
>

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