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From: Daniel Gomez <da.gomez@samsung.com>
To: David Hildenbrand <david@redhat.com>
CC: Baolin Wang <baolin.wang@linux.alibaba.com>, "akpm@linux-foundation.org"
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Subject: Re: [PATCH 0/8] add mTHP support for anonymous shmem
Thread-Topic: [PATCH 0/8] add mTHP support for anonymous shmem
Thread-Index: AQHaoTxlCSlVdbHh/EKIyyNA4DF787GNKsyAgAAqDYA=
Date: Wed, 8 May 2024 14:28:50 +0000
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References: <cover.1714978902.git.baolin.wang@linux.alibaba.com>
	<CGME20240508113934eucas1p13a3972f3f9955365f40155e084a7c7d5@eucas1p1.samsung.com>
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	<f44dc19a-e117-4418-9114-b723c5dc1178@redhat.com>

On Wed, May 08, 2024 at 01:58:19PM +0200, David Hildenbrand wrote:
> On 08.05.24 13:39, Daniel Gomez wrote:
> > On Mon, May 06, 2024 at 04:46:24PM +0800, Baolin Wang wrote:
> > > Anonymous pages have already been supported for multi-size (mTHP) all=
ocation
> > > through commit 19eaf44954df, that can allow THP to be configured thro=
ugh the
> > > sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugep=
age-XXkb/enabled'.
> > >=20
> > > However, the anonymous shared pages will ignore the anonymous mTHP ru=
le
> > > configured through the sysfs interface, and can only use the PMD-mapp=
ed
> > > THP, that is not reasonable. Many implement anonymous page sharing th=
rough
> > > mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenar=
ios,
> > > therefore, users expect to apply an unified mTHP strategy for anonymo=
us pages,
> > > also including the anonymous shared pages, in order to enjoy the bene=
fits of
> > > mTHP. For example, lower latency than PMD-mapped THP, smaller memory =
bloat
> > > than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TL=
B miss etc.
> > >=20
> > > The primary strategy is similar to supporting anonymous mTHP. Introdu=
ce
> > > a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled=
',
> > > which can have all the same values as the top-level
> > > '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a ne=
w
> > > additional "inherit" option. By default all sizes will be set to "nev=
er"
> > > except PMD size, which is set to "inherit". This ensures backward com=
patibility
> > > with the shmem enabled of the top level, meanwhile also allows indepe=
ndent
> > > control of shmem enabled for each mTHP.
> >=20
> > I'm trying to understand the adoption of mTHP and how it fits into the =
adoption
> > of (large) folios that the kernel is moving towards. Can you, or anyone=
 involved
> > here, explain this? How much do they overlap, and can we benefit from h=
aving
> > both? Is there any argument against the adoption of large folios here t=
hat I
> > might have missed?
>=20
> mTHP are implemented using large folios, just like traditional PMD-sized =
THP
> are. (you really should explore the history of mTHP and how it all works
> internally)

I'll check more in deep the code. By any chance are any of you going to be =
at
LSFMM this year? I have this session [1] scheduled for Wednesday and it wou=
ld
be nice to get your feedback on it and if you see this working together wit=
h
mTHP/THP.

[1] https://lore.kernel.org/all/4ktpayu66noklllpdpspa3vm5gbmb5boxskcj2q6qn7=
md3pwwt@kvlu64pqwjzl/

>=20
> The biggest challenge with memory that cannot be evicted on memory pressu=
re
> to be reclaimed (in contrast to your ordinary files in the pagecache) is
> memory waste, well, and placement of large chunks of memory in general,
> during page faults.
>=20
> In the worst case (no swap), you allocate a large chunk of memory once an=
d
> it will stick around until freed: no reclaim of that memory.

I can see that path being triggered by some fstests but only for THP (where=
 we
can actually reclaim memory).

>=20
> That's the reason why THP for anonymous memory and SHMEM have toggles to
> manually enable and configure them, in contrast to the pagecache. The sam=
e
> was done for mTHP for anonymous memory, and now (anon) shmem follows.
>=20
> There are plans to have, at some point, have it all working automatically=
,
> but a lot for that for anonymous memory (and shmem similarly) is still
> missing and unclear.

Thanks.

>=20
> --=20
> Cheers,
>=20
> David / dhildenb
> =