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

[Gao Xiang <hsiangkao@redhat.com>]

Hi Ying,

On Thu, Aug 20, 2020 at 12:36:08PM +0800, Huang, Ying wrote:
> Gao Xiang <hsiangkao@redhat.com> writes:
> 
> > SWP_FS doesn't mean the device is file-backed swap device,
> > which just means each writeback request should go through fs
> > by DIO. Or it'll just use extents added by .swap_activate(),
> > but it also works as file-backed swap device.
> >
> > 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.
> >
> > 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: stable <stable@vger.kernel.org>
> > Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> 
> Good catch!  The fix itself looks good me!  Although the description is
> a little confusing.
> 
> After some digging, it seems that SWP_FS is set on the swap devices
> which make swap entry read/write go through the file system specific
> callback (now used by swap over NFS only).

Okay, let me send out v2 with the updated commit message in
https://lore.kernel.org/r/20200820012409.GB5846@xiangao.remote.csb/

Thanks,
Gao Xiang

> 
> Best Regards,
> Huang, Ying
> 
> > ---
> >
> > 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
> >  ... 
> >
> >  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,
>