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

[Huang, Ying]

Hi, Zi,

Thanks for your patch!

Zi Yan <zi.yan@xxxxxxxx> writes:

> 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.
>
> Overview
> ===
>
> To support >0 order folio compaction, the patchset changes how free pages used
> for migration are kept during compaction.

migrate_pages() can split the large folio for allocation failure.  So
the minimal implementation could be

- allow to migrate large folios in compaction
- return -ENOMEM for order > 0 in compaction_alloc()

The performance may be not desirable.  But that may be a baseline for
further optimization.

And, if we can measure the performance for each step of optimization,
that will be even better.

> 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

Trivial.

s/ascending/descending/

> reduce free page split. Otherwise, high order free pages might be
> prematurely split, causing undesired high order folio migration failures.
>
>
> TODOs
> ===
>
> 1. Refactor free page post allocation and free page preparation code so
> that compaction_alloc() and compaction_free() can call functions instead
> of hard coding.
>
> 2. One possible optimization is to allow migrate_pages() to continue
> even if get_new_folio() returns a NULL. In general, that means there is
> not enough memory. But in >0 order folio compaction case, that means
> there is no suitable free page at source page order. It might be better
> to skip that page and finish the rest of migration to achieve a better
> compaction result.

We can split the source folio if get_new_folio() returns NULL.  So, do
we really need this?

In general, we may reconsider all further optimizations given splitting
is available already.

> 3. Another possible optimization is to enable free page merge. It is
> possible that a to-be-migrated page causes free page split then fails to
> migrate eventually. We would lose a high order free page without free
> page merge function. But a way of identifying free pages for memory
> compaction is needed to reuse existing PFN-based buddy page merge.
>
> 4. The implemented >0 order folio compaction algorithm is quite naive
> and does not consider all possible situations. A better algorithm can
> improve compaction success rate.
>
>
> Feel free to give comments and ask questions.
>
> Thanks.
>
>
> [1] https://lore.kernel.org/linux-mm/f8d47176-03a8-99bf-a813-b5942830fd73@xxxxxxx/
>
> Zi Yan (4):
>   mm/compaction: add support for >0 order folio memory compaction.
>   mm/compaction: optimize >0 order folio compaction with free page
>     split.
>   mm/compaction: optimize >0 order folio compaction by sorting source
>     pages.
>   mm/compaction: enable compacting >0 order folios.
>
>  mm/compaction.c | 205 +++++++++++++++++++++++++++++++++++++++---------
>  mm/internal.h   |   7 +-
>  2 files changed, 176 insertions(+), 36 deletions(-)

--
Best Regards,
Huang, Ying