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From: Barry Song <21cnbao@gmail.com>
Date: Mon, 11 Mar 2024 18:01:14 +0800
Message-ID: <CAGsJ_4xWyA1qtTR=iNukJ7=RqqJAxc2hXZSX6LtP3WbcEK5g9Q@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>, Lance Yang <ioworker0@gmail.com>, 
	Vishal Moola <vishal.moola@gmail.com>, akpm@linux-foundation.org, zokeefe@google.com, 
	shy828301@gmail.com, mhocko@suse.com, fengwei.yin@intel.com, 
	xiehuan09@gmail.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com, 
	peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org
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On Mon, Mar 11, 2024 at 5:55=E2=80=AFPM Ryan Roberts <ryan.roberts@arm.com>=
 wrote:
>
> [...]
>
> >>>>> we don't want reclamation overhead later. and we want memories imme=
diately
> >>>>> available to others.
> >>>>
> >>>> But by that logic, you also don't want to leave the large folio part=
ially mapped
> >>>> all the way until the last subpage is CoWed. Surely you would want t=
o reclaim it
> >>>> when you reach partial map status?
> >>>
> >>> To some extent, I agree. But then we will have two many copies. The l=
ast
> >>> subpage is small, and a safe place to copy instead.
> >>>
> >>> We actually had to tune userspace to decrease partial map as too much
> >>> partial map both unfolded CONT-PTE and wasted too much memory. if a
> >>> vma had too much partial map, we disabled mTHP on this VMA.
> >>
> >> I actually had a whacky idea around introducing selectable page size A=
BI
> >> per-process that might help here. I know Android is doing work to make=
 the
> >> system 16K page compatible. You could run most of the system processes=
 with 16K
> >> ABI on top of 4K kernel. Then those processes don't even have the abil=
ity to
> >> madvise/munmap/mprotect/mremap anything less than 16K alignment so tha=
t acts as
> >> an anti-fragmentation mechanism while allowing non-16K capable process=
es to run
> >> side-by-side. Just a passing thought...
> >
> > Right, this project faces a challenge in supporting legacy
> > 4KiB-aligned applications.
> > but I don't find it will be an issue to run 16KiB-aligned applications
> > on a kernel whose
> > page size is 4KiB.
>
> Yes, agreed that a 16K-aligned (or 64K-aligned) app will work without iss=
ue on
> 4K kernel, but it will also use getpagesize() and know what the page size=
 is.
> I'm suggesting you could actually run these apps on a 4K kernel but with =
a 16K
> ABI and potentially get close to the native 16K performance out of them. =
It's
> just a thought though - I don't have any data that actually shows this is=
 better
> than just running on a 4K kernel with a 4K ABI, and using 16K or 64K mTHP
> opportunistically.

 I fully agree with this as my Ubuntu filesystem can run on 4KiB, 16KiB and
64KiB basepage size as its elf files are 64KiB aligned. so I would expect
new Android apps/middleware move to 64KiB ABI though it might want to
change the base page size to 16KiB instead.
I believe this is the case.

Thanks
Barry