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

[Matthew Wilcox <willy@infradead.org>]

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.

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