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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Yang Shi <shy828301@gmail.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v2 0/5] variable-order, large folios for anonymous memory
Date:   Mon, 10 Jul 2023 12:53:33 -0400
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On 7 Jul 2023, at 9:24, David Hildenbrand wrote:

> On 07.07.23 15:12, Matthew Wilcox wrote:
>> On Fri, Jul 07, 2023 at 01:40:53PM +0200, David Hildenbrand wrote:
>>> On 06.07.23 10:02, Ryan Roberts wrote:
>>> But can you comment on the page migration part (IOW did you try it al=
ready)?
>>>
>>> For example, memory hotunplug, CMA, MCE handling, compaction all rely=
 on
>>> page migration of something that was allocated using GFP_MOVABLE to a=
ctually
>>> work.
>>>
>>> Compaction seems to skip any higher-order folios, but the question is=
 if the
>>> udnerlying migration itself works.
>>>
>>> If it already works: great! If not, this really has to be tackled ear=
ly,
>>> because otherwise we'll be breaking the GFP_MOVABLE semantics.
>>
>> I have looked at this a bit.  _Migration_ should be fine.  _Compaction=
_
>> is not.
>
> Thanks! Very nice if at least ordinary migration works.
>
>>
>> If you look at a function like folio_migrate_mapping(), it all seems
>> appropriately folio-ised.  There might be something in there that is
>> slightly wrong, but that would just be a bug to fix, not a huge
>> architectural problem.
>>
>> The problem comes in the callers of migrate_pages().  They pass a
>> new_folio_t callback.  alloc_migration_target() is the usual one passe=
d
>> and as far as I can tell is fine.  I've seen no problems reported with=
 it.
>>
>> compaction_alloc() is a disaster, and I don't know how to fix it.
>> The compaction code has its own allocator which is populated with orde=
r-0
>> folios.  How it populates that freelist is awful ... see split_map_pag=
es()
>
> Yeah, all that code was written under the assumption that we're moving =
order-0 pages (which is what the anon+pagecache pages part).
>
> From what I recall, we're allocating order-0 pages from the high memory=
 addresses, so we can migrate from low memory addresses, effectively free=
ing up low memory addresses and filling high memory addresses.
>
> Adjusting that will be ... interesting. Instead of allocating order-0 p=
ages from high addresses, we might want to allocate "as large as possible=
" ("grab what we can") from high addresses and then have our own kind of =
buddy for allocating from that pool a compaction destination page, depend=
ing on our source page. Nasty.

We probably do not need a pool, since before migration, we have isolated =
folios to
be migrated and can come up with a stats on how many folios there are at =
each order.
Then, we can isolate free pages based on the stats and do not split free =
pages
all the way down to order-0. We can sort the source folios based on their=
 orders
and isolate free pages from largest order to smallest order. That could a=
void
a free page pool.

--
Best Regards,
Yan, Zi

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