linux-kernel - Re: [PATCH v5 3/4] mm: support large folios swapin as a whole for zRAM-like swapfile

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Message-ID: <CAGsJ_4wxUZAysyg3cCVnHhOFt5SbyAMUfq3tJcX-Wb6D4BiBhA@mail.gmail.com>
Date: Mon, 29 Jul 2024 16:46:42 +1200
From: Barry Song <21cnbao@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, ying.huang@...el.com, 
	baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com, 
	hannes@...xchg.org, hughd@...gle.com, kaleshsingh@...gle.com, 
	kasong@...cent.com, linux-kernel@...r.kernel.org, mhocko@...e.com, 
	minchan@...nel.org, nphamcs@...il.com, ryan.roberts@....com, 
	senozhatsky@...omium.org, shakeel.butt@...ux.dev, shy828301@...il.com, 
	surenb@...gle.com, v-songbaohua@...o.com, xiang@...nel.org, 
	yosryahmed@...gle.com, Chuanhua Han <hanchuanhua@...o.com>
Subject: Re: [PATCH v5 3/4] mm: support large folios swapin as a whole for
 zRAM-like swapfile

On Mon, Jul 29, 2024 at 4:41 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Mon, Jul 29, 2024 at 3:51 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Fri, Jul 26, 2024 at 09:46:17PM +1200, Barry Song wrote:
> > > -                     folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > > -                                             vma, vmf->address, false);
> > > +                     folio = alloc_swap_folio(vmf);
> > >                       page = &folio->page;
> >
> > This is no longer correct.  You need to set 'page' to the precise page
> > that is being faulted rather than the first page of the folio.  It was
> > fine before because it always allocated a single-page folio, but now it
> > must use folio_page() or folio_file_page() (whichever has the correct
> > semantics for you).
> >
> > Also you need to fix your test suite to notice this bug.  I suggest
> > doing that first so that you know whether you've got the calculation
> > correct.
>
> I don't understand why the code is designed in the way the page
> is the first page of this folio. Otherwise, we need lots of changes
> later while mapping the folio in ptes and rmap.

For both accessing large folios in the swapcache and allocating
new large folios, the page points to the first page of the folio. we
are mapping the whole folio not the specific page.

for swapcache cases, you can find the same thing here,

        if (folio_test_large(folio) && folio_test_swapcache(folio)) {
                ...
                entry = folio->swap;
                page = &folio->page;
        }

Thanks
Barry