Re: [RFC PATCH 0/4] Enable >0 order folio memory compaction

[Zi Yan <ziy@nvidia.com>]

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On 9 Oct 2023, at 10:10, Ryan Roberts wrote:

> On 09/10/2023 14:24, Zi Yan wrote:
>> On 2 Oct 2023, at 8:32, Ryan Roberts wrote:
>>
>>> Hi Zi,
>>>
>>> On 12/09/2023 17:28, Zi Yan wrote:
>>>> From: Zi Yan <ziy@nvidia.com>
>>>>
>>>> Hi all,
>>>>
>>>> This patchset enables >0 order folio memory compaction, which is one of
>>>> the prerequisitions for large folio support[1]. It is on top of
>>>> mm-everything-2023-09-11-22-56.
>>>
>>> I've taken a quick look at these and realize I'm not well equipped to provide
>>> much in the way of meaningful review comments; All I can say is thanks for
>>> putting this together, and yes, I think it will become even more important for
>>> my work on anonymous large folios.
>>>
>>>
>>>>
>>>> Overview
>>>> ===
>>>>
>>>> To support >0 order folio compaction, the patchset changes how free pages used
>>>> for migration are kept during compaction. Free pages used to be split into
>>>> order-0 pages that are post allocation processed (i.e., PageBuddy flag cleared,
>>>> page order stored in page->private is zeroed, and page reference is set to 1).
>>>> Now all free pages are kept in a MAX_ORDER+1 array of page lists based
>>>> on their order without post allocation process. When migrate_pages() asks for
>>>> a new page, one of the free pages, based on the requested page order, is
>>>> then processed and given out.
>>>>
>>>>
>>>> Optimizations
>>>> ===
>>>>
>>>> 1. Free page split is added to increase migration success rate in case
>>>> a source page does not have a matched free page in the free page lists.
>>>> Free page merge is possible but not implemented, since existing
>>>> PFN-based buddy page merge algorithm requires the identification of
>>>> buddy pages, but free pages kept for memory compaction cannot have
>>>> PageBuddy set to avoid confusing other PFN scanners.
>>>>
>>>> 2. Sort source pages in ascending order before migration is added to
>>>> reduce free page split. Otherwise, high order free pages might be
>>>> prematurely split, causing undesired high order folio migration failures.
>>>
>>> Not knowing much about how compaction actually works, naively I would imagine
>>> that if you are just trying to free up a known amount of contiguous physical
>>> space, then working through the pages in PFN order is more likely to yield the
>>> result quicker? Unless all of the pages in the set must be successfully migrated
>>> in order to free up the required amount of space...
>>
>> During compaction, pages are not freed, since that is the job of page reclaim.
>
> Sorry yes - my fault for using sloppy language. When I said "free up a known
> amount of contiguous physical space", I really meant "move pages in order to
> recover an amount of contiguous physical space". But I still think the rest of
> what I said applies; wouldn't you be more likely to reach your goal quicker if
> you sort by PFN?

Not always. If the in-use folios on the left are order-2, order-2, order-4
(all contiguous in one pageblock) and free pages on the right are order-4 (pageblock N),
order-2, order-2 (pageblock N-1) and it is not a single order-8, since there are
in-use folios in the middle), going in PFN order will not get you an order-8 free
page, since first order-4 free page will be split into two order-2 for the first
two order-2 in-use folios. But if you migrate in the the descending order of
in-use page orders, you can get an order-8 free page at the end.

The patcheset minimizes free page splits to avoid the situation described above,
since once a high order free page is split, the opportunity of migrating a high order
in-use folio into it is gone and hardly recoverable.

>
>> The goal of compaction is to get a high order free page without freeing existing
>> pages to avoid potential high cost IO operations. If compaction does not work,
>> page reclaim would free pages to get us there (and potentially another follow-up
>> compaction). So either pages are migrated or stay where they are during compaction.
>>
>> BTW compaction works by scanning in use pages from lower PFN to higher PFN,
>> and free pages from higher PFN to lower PFN until two scanners meet in the middle.
>>
>> --
>> Best Regards,
>> Yan, Zi

Best Regards,
Yan, Zi

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