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Message-ID: <63d119f7-5adb-861a-00c2-69a92b19ef9b@redhat.com>
Date:   Thu, 12 Oct 2023 10:33:04 +0200
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Subject: Re: [PATCH v1 0/5] mm, kpageflags: support folio and fix output for
 compound pages
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To:     Naoya Horiguchi <naoya.horiguchi@linux.dev>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Muchun Song <songmuchun@bytedance.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        linux-kernel@vger.kernel.org
References: <20231010142801.3780917-1-naoya.horiguchi@linux.dev>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20231010142801.3780917-1-naoya.horiguchi@linux.dev>
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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.

> 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?

-- 
Cheers,

David / dhildenb