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

[Ryan Roberts]

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@xxxxxxxxxx>
>>>
>>> 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?

> 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