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

[Gao Xiang <hsiangkao@redhat.com>]

Hi Yang,

On Wed, Aug 19, 2020 at 02:41:08PM -0700, Yang Shi wrote:
> On Wed, Aug 19, 2020 at 1:15 PM Gao Xiang <hsiangkao@redhat.com> wrote:
> >
> > Hi Andrew,
> >
> > On Wed, Aug 19, 2020 at 01:05:06PM -0700, Andrew Morton wrote:
> > > On Thu, 20 Aug 2020 03:56:13 +0800 Gao Xiang <hsiangkao@redhat.com> wrote:
> > >
> > > > 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.
> > >
> > > This is very hard to understand :(
> >
> > Thanks for your reply...
> >
> > The related logic is in __swap_writepage() and setup_swap_extents(),
> > and also see e.g generic_swapfile_activate() or iomap_swapfile_activate()...
> 
> I think just NFS falls into this case, so you may rephrase it to:
> 
> SWP_FS is only used for swap files over NFS. So, !SWP_FS means non NFS
> swap, it could be either file backed or device backed.

Thanks for your suggestion...

That looks reasonable, and after I looked
bc4ae27d817a ("mm: split SWP_FILE into SWP_ACTIVATED and SWP_FS")

I think it could be rephrased into

"
The SWP_FS flag 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.
"

Does it look sane? And I will wait for further suggestion
about this for a while.

And IMO, SWP_FS flag might be useful for other uses later
(e.g. laterly for some CoW swapfile use, but I don't think
 carefully if it's practical or not...)

Thanks,
Gao Xiang