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From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 31 Oct 2023 11:29:34 -0700
Message-ID: <CAHbLzkrJAsWzAJCrh+Dh5gNTE1UrH9ndbWQqH9fStoptpMkEew@mail.gmail.com>
Subject: Re: [PATCH v6 0/9] variable-order, large folios for anonymous memory
To:     Ryan Roberts <ryan.roberts@arm.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Itaru Kitayama <itaru.kitayama@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
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On Tue, Oct 31, 2023 at 4:55=E2=80=AFAM Ryan Roberts <ryan.roberts@arm.com>=
 wrote:
>
> On 31/10/2023 11:50, Ryan Roberts wrote:
> > 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_hugep=
age/", 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 somethi=
ng 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 every=
thing 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 dr=
ive
> > decisions for both anonymous memory and (non-shmem) file-backed memory.=
 But the
> > proposed new per-size "enabled" is implicitly only controlling anon mem=
ory (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 f=
or 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 dec=
ision
> > (for now), even when hugepages-2048kB/enabled !=3D "global"; agreed?
> >
> > Thanks,
> > Ryan
> >
>
> Also, an implementation question:
>
> hugepage_vma_check() doesn't currently care whether enabled=3D"never" for=
 DAX VMAs
> (although it does honour MADV_NOHUGEPAGE and the prctl); It will return t=
rue
> regardless. Is that by design? It couldn't fathom any reasoning from the =
commit log:

The enabled=3D"never" is for anonymous VMAs, DAX VMAs are typically file VM=
As.

>
> bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flag=
s,
>                         bool smaps, bool in_pf, bool enforce_sysfs)
> {
>         if (!vma->vm_mm)                /* vdso */
>                 return false;
>
>         /*
>          * Explicitly disabled through madvise or prctl, or some
>          * architectures may disable THP for some mappings, for
>          * example, s390 kvm.
>          * */
>         if ((vm_flags & VM_NOHUGEPAGE) ||
>             test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>                 return false;
>         /*
>          * If the hardware/firmware marked hugepage support disabled.
>          */
>         if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_UNSUP=
PORTED))
>                 return false;
>
>         /* khugepaged doesn't collapse DAX vma, but page fault is fine. *=
/
>         if (vma_is_dax(vma))
>                 return in_pf;  <<<<<<<<
>
>         ...
> }
>
>