Re: [PATCH 1/2] MM: handle THP in swap_*page_fs()

[NeilBrown]

On Tue, 03 May 2022, Yang Shi wrote:
> On Sun, May 1, 2022 at 9:23 PM NeilBrown <neilb@xxxxxxx> wrote:
> >
> > On Sat, 30 Apr 2022, Yang Shi wrote:
> > > On Thu, Apr 28, 2022 at 5:44 PM NeilBrown <neilb@xxxxxxx> wrote:
> > > >
> > > > Pages passed to swap_readpage()/swap_writepage() are not necessarily all
> > > > the same size - there may be transparent-huge-pages involves.
> > > >
> > > > The BIO paths of swap_*page() handle this correctly, but the SWP_FS_OPS
> > > > path does not.
> > > >
> > > > So we need to use thp_size() to find the size, not just assume
> > > > PAGE_SIZE, and we need to track the total length of the request, not
> > > > just assume it is "page * PAGE_SIZE".
> > >
> > > Swap-over-nfs doesn't support THP swap IIUC. So SWP_FS_OPS should not
> > > see THP at all. But I agree to remove the assumption about page size
> > > in this path.
> >
> > Can you help me understand this please.  How would the swap code know
> > that swap-over-NFS doesn't support THP swap?  There is no reason that
> > NFS wouldn't be able to handle 2MB writes.  Even 1GB should work though
> > NFS would have to split into several smaller WRITE requests.
> 
> AFAICT, THP swap is only supported on non-rotate block devices, for
> example, SSD, PMEM, etc. IIRC, the swap device has to support the
> cluster in order to swap THP. The cluster is only supported by
> non-rotate block devices.
> 
> Looped Ying in, who is the author of THP swap.

I hunted around the code and found that THP swap only happens if a
'cluster_info' is allocated, and that only happens if 
	if (p->bdev && bdev_nonrot(p->bdev)) {
in the swapon syscall.

I guess "nonrot" is being use as a synonym for "low latency"...
So even if NFS was low-latency it couldn't benefit from THP swap.

So as you say it is not currently possible for THP pages to be send to
NFS for swapout.  It makes sense to prepare for it though I think - if
only so that the code is more consistent and less confusing.

Thanks,
NeilBrown