[PATCH v2] mm, THP, swap: fix allocating cluster for swapfile by mistake

[Gao Xiang <hsiangkao@redhat.com>]

SWP_FS is used to make swap_{read,write}page() go through
the filesystem, and it's only used for swap files over
NFS. So, !SWP_FS means non NFS for now, it could be either
file backed or device backed. Something similar goes with
legacy SWP_FILE.

So in order to achieve the goal of the original patch,
SWP_BLKDEV should be used instead.

FS corruption can be observed with SSD device + XFS +
fragmented swapfile due to CONFIG_THP_SWAP=y.

I reproduced the issue with the following details:

Environment:
QEMU + upstream kernel + buildroot + NVMe (2 GB)

Kernel config:
CONFIG_BLK_DEV_NVME=y
CONFIG_THP_SWAP=y

Some reproducable steps:
mkfs.xfs -f /dev/nvme0n1
mkdir /tmp/mnt
mount /dev/nvme0n1 /tmp/mnt
bs="32k"
sz="1024m"    # doesn't matter too much, I also tried 16m
xfs_io -f -c "pwrite -R -b $bs 0 $sz" -c "fdatasync" /tmp/mnt/sw
xfs_io -f -c "pwrite -R -b $bs 0 $sz" -c "fdatasync" /tmp/mnt/sw
xfs_io -f -c "pwrite -R -b $bs 0 $sz" -c "fdatasync" /tmp/mnt/sw
xfs_io -f -c "pwrite -F -S 0 -b $bs 0 $sz" -c "fdatasync" /tmp/mnt/sw
xfs_io -f -c "pwrite -R -b $bs 0 $sz" -c "fsync" /tmp/mnt/sw

mkswap /tmp/mnt/sw
swapon /tmp/mnt/sw

stress --vm 2 --vm-bytes 600M   # doesn't matter too much as well

Symptoms:
 - FS corruption (e.g. checksum failure)
 - memory corruption at: 0xd2808010
 - segfault

Fixes: f0eea189e8e9 ("mm, THP, swap: Don't allocate huge cluster for file backed swap device")
Fixes: 38d8b4e6bdc8 ("mm, THP, swap: delay splitting THP during swap out")
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
---
v1: https://lore.kernel.org/r/20200819195613.24269-1-hsiangkao@redhat.com

changes since v1:
 - improve commit message description

Hi Andrew,
Kindly consider this one instead if no other concerns...

Thanks,
Gao Xiang

 mm/swapfile.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 6c26916e95fd..2937daf3ca02 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1074,7 +1074,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
 			goto nextsi;
 		}
 		if (size == SWAPFILE_CLUSTER) {
-			if (!(si->flags & SWP_FS))
+			if (si->flags & SWP_BLKDEV)
 				n_ret = swap_alloc_cluster(si, swp_entries);
 		} else
 			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
-- 
2.18.1