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

[Rafael Aquini]

On Thu, Aug 20, 2020 at 12:53:23PM +0800, Gao Xiang wrote:
> 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@xxxxxxxxx>
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: Rafael Aquini <aquini@xxxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: stable <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>
> ---
> v1: https://lore.kernel.org/r/20200819195613.24269-1-hsiangkao@xxxxxxxxxx
> 
> 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
> 
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>