Re: [PATCH] MM: handle THP in swap_*page_fs() - count_vm_events()

[Yang Shi <shy828301@gmail.com>]

On Sun, May 1, 2022 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, May 02, 2022 at 03:28:49PM +1000, NeilBrown wrote:
> > On Mon, 02 May 2022, Matthew Wilcox wrote:
> > > On Mon, May 02, 2022 at 02:57:46PM +1000, NeilBrown wrote:
> > > > @@ -390,9 +392,9 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
> > > >                   struct page *page = sio->bvec[p].bv_page;
> > > >
> > > >                   SetPageUptodate(page);
> > > > +                 count_swpout_vm_event(page);
> > > >                   unlock_page(page);
> > > >           }
> > > > -         count_vm_events(PSWPIN, sio->pages);
> > >
> > > Surely that should be count_swpIN_vm_event?
> > >
> > I'm not having a good day....
> >
> > Certainly shouldn't be swpout.  There isn't a count_swpin_vm_event().
> >
> > swap_readpage() only counts once for each page no matter how big it is.
> > While swap_writepage() counts one for each PAGE_SIZE written.
> >
> > And we have THP_SWPOUT but not THP_SWPIN
>
> _If_ I understand the swap-in patch correctly (at least as invoked by
> shmem), it won't attempt to swap in an entire THP.  Even if it swapped
> out an order-9 page, it will bring in order-0 pages from swap, and then
> rely on khugepaged to reassemble them.

Totally correct. The try_to_unmap() called by vmscan would split PMD
to PTEs then install swap entries for each PTE but keep the huge page
unsplit.

BTW, there were patches adding THP swapin support, but they were never merged.

>
> Someone who actually understands the swap code should check that my
> explanation here is correct.
>