mm: compaction: push watermark into compaction_suitable() callers

Patch series "mm: reliable huge page allocator".

This series makes changes to the allocator and reclaim/compaction code to
try harder to avoid fragmentation.  As a result, this makes huge page
allocations cheaper, more reliable and more sustainable.

It's a subset of the huge page allocator RFC initially proposed here:

  https://lore.kernel.org/lkml/20230418191313.268131-1-hannes@cmpxchg.org/

The following results are from a kernel build test, with additional
concurrent bursts of THP allocations on a memory-constrained system. 
Comparing before and after the changes over 15 runs:

                                                     before                   after
    Hugealloc Time mean               52739.45 (    +0.00%)   28904.00 (   -45.19%)
    Hugealloc Time stddev             56541.26 (    +0.00%)   33464.37 (   -40.81%)
    Kbuild Real time                    197.47 (    +0.00%)     196.59 (    -0.44%)
    Kbuild User time                   1240.49 (    +0.00%)    1231.67 (    -0.71%)
    Kbuild System time                   70.08 (    +0.00%)      59.10 (   -15.45%)
    THP fault alloc                   46727.07 (    +0.00%)   63223.67 (   +35.30%)
    THP fault fallback                21910.60 (    +0.00%)    5412.47 (   -75.29%)
    Direct compact fail                 195.80 (    +0.00%)      59.07 (   -69.48%)
    Direct compact success                7.93 (    +0.00%)       2.80 (   -57.46%)
    Direct compact success rate %         3.51 (    +0.00%)       3.99 (   +10.49%)
    Compact daemon scanned migrate  3369601.27 (    +0.00%) 2267500.33 (   -32.71%)
    Compact daemon scanned free     5075474.47 (    +0.00%) 2339773.00 (   -53.90%)
    Compact direct scanned migrate   161787.27 (    +0.00%)   47659.93 (   -70.54%)
    Compact direct scanned free      163467.53 (    +0.00%)   40729.67 (   -75.08%)
    Compact total migrate scanned   3531388.53 (    +0.00%) 2315160.27 (   -34.44%)
    Compact total free scanned      5238942.00 (    +0.00%) 2380502.67 (   -54.56%)
    Alloc stall                        2371.07 (    +0.00%)     638.87 (   -73.02%)
    Pages kswapd scanned            2160926.73 (    +0.00%) 4002186.33 (   +85.21%)
    Pages kswapd reclaimed           533191.07 (    +0.00%)  718577.80 (   +34.77%)
    Pages direct scanned             400450.33 (    +0.00%)  355172.73 (   -11.31%)
    Pages direct reclaimed            94441.73 (    +0.00%)   31162.80 (   -67.00%)
    Pages total scanned             2561377.07 (    +0.00%) 4357359.07 (   +70.12%)
    Pages total reclaimed            627632.80 (    +0.00%)  749740.60 (   +19.46%)
    Swap out                          47959.53 (    +0.00%)  110084.33 (  +129.53%)
    Swap in                            7276.00 (    +0.00%)   24457.00 (  +236.10%)
    File refaults                    138043.00 (    +0.00%)  188226.93 (   +36.35%)

THP latencies are cut in half, and failure rates are cut by 75%.  These
metrics also hold up over time, while the vanilla kernel sees a steady
downward trend in success rates with each subsequent run, owed to the
cumulative effects of fragmentation.

A more detailed discussion of results is in the patch changelogs.

The patches first introduce a vm.defrag_mode sysctl, which enforces the
existing ALLOC_NOFRAGMENT alloc flag until after reclaim and compaction
have run.  They then change kswapd and kcompactd to target pageblocks,
which boosts success in the ALLOC_NOFRAGMENT hotpaths.

Patches #1 and #2 are somewhat unrelated cleanups, but touch the same code
and so are included here to avoid conflicts from re-ordering.


This patch (of 5):

compaction_suitable() hardcodes the min watermark, with a boost to the low
watermark for costly orders.  However, compaction_ready() requires order-0
at the high watermark.  It currently checks the marks twice.

Make the watermark a parameter to compaction_suitable() and have the
callers pass in what they require:

- compaction_zonelist_suitable() is used by the direct reclaim path,
  so use the min watermark.

- compact_suit_allocation_order() has a watermark in context derived
  from cc->alloc_flags.

  The only quirk is that kcompactd doesn't initialize cc->alloc_flags
  explicitly. There is a direct check in kcompactd_do_work() that
  passes ALLOC_WMARK_MIN, but there is another check downstack in
  compact_zone() that ends up passing the unset alloc_flags. Since
  they default to 0, and that coincides with ALLOC_WMARK_MIN, it is
  correct. But it's subtle. Set cc->alloc_flags explicitly.

- should_continue_reclaim() is direct reclaim, use the min watermark.

- Finally, consolidate the two checks in compaction_ready() to a
  single compaction_suitable() call passing the high watermark.

  There is a tiny change in behavior: before, compaction_suitable()
  would check order-0 against min or low, depending on costly
  order. Then there'd be another high watermark check.

  Now, the high watermark is passed to compaction_suitable(), and the
  costly order-boost (low - min) is added on top. This means
  compaction_ready() sets a marginally higher target for free pages.

  In a kernelbuild + THP pressure test, though, this didn't show any
  measurable negative effects on memory pressure or reclaim rates. As
  the comment above the check says, reclaim is usually stopped short
  on should_continue_reclaim(), and this just defines the worst-case
  reclaim cutoff in case compaction is not making any headway.

[hughd@google.com: stop oops on out-of-range highest_zoneidx]
  Link: https://lkml.kernel.org/r/005ace8b-07fa-01d4-b54b-394a3e029c07@google.com
Link: https://lkml.kernel.org/r/20250313210647.1314586-1-hannes@cmpxchg.org
Link: https://lkml.kernel.org/r/20250313210647.1314586-2-hannes@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>