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-and-tested-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/swapfile.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 58877312cf6b..e55a69fd24e4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1970,9 +1970,9 @@ 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);
@@ -2197,7 +2197,6 @@ 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
@@ -2426,12 +2425,15 @@ 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;
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;
@@ -2574,10 +2576,8 @@ 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:
@@ -2587,8 +2587,6 @@ 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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