Re: [PATCH] nfs: fix nfs_swap_rw for large-folio swap

[Ryan Roberts <ryan.roberts@arm.com>]

On 16/06/2024 11:23, Barry Song wrote:
> On Sun, Jun 16, 2024 at 4:54 PM Christoph Hellwig <hch@lst.de> wrote:
>>
>> On Sun, Jun 16, 2024 at 12:16:10PM +1200, Barry Song wrote:
>>> As I understand it, this isn't happening because we don't support
>>> mTHP swapping out to a swapfile, whether it's on NFS or any
>>> other filesystem.
>>
>> It does happen.  The reason why I sent this patch is becaue I observed
>> the BUG_ON trigger on a trivial swap generation workload (usemem.c from
>> xfstests).
> 
> This is quite unusual. Could you share your setup and backtrace? I'd
> like to reproduce the issue, as the mm code only supports mTHP
> swapout on block devices. What is your swap device or swap file?
> Additionally, on what kind of filesystem is the executable file built
> from usemem.c located?

Yes, I'm also confused by this, since as Barry says, the swap-out changes to
support mTHP are only intended to be activated when the swap device is a
non-rotating block device - swap files on file systems are explicitly not
supported and all swapping should be done page-by-page in that case. This
constraint is exactly the same as for the pre-existing PMD-size THP swap-out
support. So if you are seeing large folios being written after the mTHP swap-out
change, you should also be seeing large folios before this change.

Hopefully the stack trace will tell us what's going on here.

(Sorry for my slow responses/lack of engagement over the last month; its been a
combination of paternity leave/lack of sleep/working on other things. I'm hoping
to get properly back into this stuff within the next couple of weeks).

Thanks,
Ryan