linux-kernel - Re: [PATCH v6 0/9] variable-order, large folios for anonymous memory

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Message-ID: <7a3a2d49-528d-4297-ae19-56aa9e6c59c6@arm.com>
Date:   Tue, 31 Oct 2023 11:50:34 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 0/9] variable-order, large folios for anonymous memory

On 06/10/2023 21:06, David Hildenbrand wrote:
[...]
> 
> Change 2: sysfs interface.
> 
> If we call it THP, it shall go under "/sys/kernel/mm/transparent_hugepage/", I
> agree.
> 
> What we expose there and how, is TBD. Again, not a friend of "orders" and
> bitmaps at all. We can do better if we want to go down that path.
> 
> Maybe we should take a look at hugetlb, and how they added support for multiple
> sizes. What *might* make sense could be (depending on which values we actually
> support!)
> 
> 
> /sys/kernel/mm/transparent_hugepage/hugepages-64kB/
> /sys/kernel/mm/transparent_hugepage/hugepages-128kB/
> /sys/kernel/mm/transparent_hugepage/hugepages-256kB/
> /sys/kernel/mm/transparent_hugepage/hugepages-512kB/
> /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/
> /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/
> 
> Each one would contain an "enabled" and "defrag" file. We want something minimal
> first? Start with the "enabled" option.
> 
> 
> enabled: always [global] madvise never
> 
> Initially, we would set it for PMD-sized THP to "global" and for everything else
> to "never".

Hi David,

I've just started coding this, and it occurs to me that I might need a small
clarification here; the existing global "enabled" control is used to drive
decisions for both anonymous memory and (non-shmem) file-backed memory. But the
proposed new per-size "enabled" is implicitly only controlling anon memory (for
now).

1) Is this potentially confusing for the user? Should we rename the per-size
controls to "anon_enabled"? Or is it preferable to jsut keep it vague for now so
we can reuse the same control for file-backed memory in future?

2) The global control will continue to drive the file-backed memory decision
(for now), even when hugepages-2048kB/enabled != "global"; agreed?

Thanks,
Ryan