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

[Zi Yan <zi.yan@sent.com>]

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.

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.


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.

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@arm.com/

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(-)

-- 
2.40.1