linux-kernel - Re: [PATCH v1 0/5] mm, kpageflags: support folio and fix output for compound pages

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Date:   Fri, 13 Oct 2023 00:02:26 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/5] mm, kpageflags: support folio and fix output for
 compound pages

On Thu, Oct 12, 2023 at 10:33:04AM +0200, David Hildenbrand wrote:
> On 10.10.23 16:27, Naoya Horiguchi wrote:
> > Hi everyone,
> > 
> > This patchset addresses 2 issues in /proc/kpageflags.
> > 
> >    1. We can't easily tell folio from thp, because currently both pages are
> >       judged as thp, and
> >    2. we see some garbage data in records of compound tail pages because
> >       we use tail pages to store some internal data.
> > 
> > These issues require userspace programs to do additional work to understand
> > the page status, which makes situation more complicated.
> > 
> > This patchset tries to solve these by defining KPF_FOLIO for issue 1., and
> > by hiding part of page flag info on tail pages of compound pages for issue 2.
> > 
> > I think that technically some compound pages like thp/hugetlb/slab could be
> > considered as folio, but in this version KPF_FOLIO is set only on folios
> 
> At least thp+hugetlb are most certainly folios. Regarding slab, I suspect we
> no longer call them folios (cannot be mapped to user space). But Im not sure
> about the type hierarchy.

I'm not sure about the exact definition of "folio", and I think it's better
to make KPF_FOLIO set based on the definition.
"being mapped to userspace" can be one possible criteria for the definition.
But reading source code, folio_slab() and slab_folio() convert between
struct slab and struct folio, so I feel that someone might think a slab is
a kind of folio.

> 
> > in pagecache (so "folios in narrower meaning").  I'm not confident about
> > this choice, so if you have any idea about this, please let me know.
> 
> It does sound inconsistent. What exactly do you want to tell user space with
> the new flag?

The current most problematic behavior is to report folio as thp (order-2
pagecache page is definitely a folio but not a thp), and this is what the
new flag is intended to tell.

Thanks,
Naoya Horiguchi