Re: [PATCH v2 0/5] variable-order, large folios for anonymous memory

[Zi Yan]

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 already)?
>>>
>>> For example, memory hotunplug, CMA, MCE handling, compaction all rely on
>>> page migration of something that was allocated using GFP_MOVABLE to actually
>>> 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 early,
>>> 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 passed
>> 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 order-0
>> folios.  How it populates that freelist is awful ... see split_map_pages()
>
> 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 freeing up low memory addresses and filling high memory addresses.
>
> Adjusting that will be ... interesting. Instead of allocating order-0 pages 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, depending 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 avoid
a free page pool.

--
Best Regards,
Yan, Zi

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