[PATCH 0/3] mm: make watermark checks honour ALLOC_NOFRAGMENT

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

With defrag_mode, 64K mTHP faults under memory pressure stall in
compaction 18 times as often as without it, and get about a third of the
folios.  Dev Jain reported it [1].

The high-order watermark check counts free pages of every migratetype.
With defrag_mode, every slowpath allocation is ALLOC_NOFRAGMENT, and a
request below pageblock_order cannot take free pages from another
migratetype's pageblock.  Under pressure, that is where the large free
pages end up.  Two callers act on the wrong answer:

  - direct compaction's up-front check returns COMPACT_SUCCESS without
    compacting, so compaction is never deferred;
  - direct reclaim for a costly order stops early, having freed little.

Patches 1-2 give the check the request's migratetype and make it skip
the free lists the request cannot take.  Patch 3 tells direct reclaim
that the allocation is ALLOC_NOFRAGMENT.

Dev's benchmark in a 7G VM with only 64K mTHP enabled, medians of 15
runs:

                              defrag_mode=0  defrag_mode=1  defrag_mode=1
                                                            + series
    compact_stall                      1690          30669          7366
    64K faults allocated              20750           7391         20129
    64K faults falling back to 4K       725          13832          1339

defrag_mode=1 still compacts more than defrag_mode=0: by design, it
keeps 64K faults out of unmovable and reclaimable pageblocks [2].
defrag_mode=0 is unchanged: 20698 64K faults allocated with the series.

[1] https://lore.kernel.org/all/4aa14b41-9b89-494c-8825-55fffd5a541c@xxxxxxx/
[2] https://lore.kernel.org/all/auCDnqJc7fsBpUW2@thinkstation/

Kiryl Shutsemau (Meta) (3):
  mm: page_alloc: add a watermark check that knows the migratetype
  mm: page_alloc: make the high-order watermark check honour
    ALLOC_NOFRAGMENT
  mm: vmscan: tell direct reclaim the allocation is ALLOC_NOFRAGMENT

 include/linux/mmzone.h |  3 --
 mm/compaction.c        | 16 +++++++----
 mm/internal.h          | 10 ++++++-
 mm/page_alloc.c        | 64 +++++++++++++++++++++++++++++++++---------
 mm/vmscan.c            | 24 ++++++++++++----
 5 files changed, 89 insertions(+), 28 deletions(-)


base-commit: d865f2677c44cb2171b833d6775cf692756fa8be
-- 
2.54.0