mm/page_alloc: keep track of free highatomic

mm/page_alloc: keep track of free highatomic

OOM kills due to vastly overestimated free highatomic reserves were
observed:

  ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ...
  Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ...
  Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB

The second line above shows that the OOM kill was due to the following
condition:

  free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB)

And the third line shows there were no free pages in any
MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type 'H'.
Therefore __zone_watermark_unusable_free() underestimated the usable free
memory by over 1GB, which resulted in the unnecessary OOM kill above.

The comments in __zone_watermark_unusable_free() warns about the potential
risk, i.e.,

  If the caller does not have rights to reserves below the min
  watermark then subtract the high-atomic reserves. This will
  over-estimate the size of the atomic reserve but it avoids a search.

However, it is possible to keep track of free pages in reserved highatomic
pageblocks with a new per-zone counter nr_free_highatomic protected by the
zone lock, to avoid a search when calculating the usable free memory.  And
the cost would be minimal, i.e., simple arithmetics in the highatomic
alloc/free/move paths.

Note that since nr_free_highatomic can be relatively small, using a
per-cpu counter might cause too much drift and defeat its purpose, in
addition to the extra memory overhead.

Dependson e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting") - see [1]

[[email protected]: s/if/else if/, per Johannes, stealth whitespace tweak]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0aaa29a56e4f ("mm, page_alloc: reserve pageblocks for high-order atomic allocations on demand")
Signed-off-by: Yu Zhao <[email protected]>
Reported-by: Link Lin <[email protected]>
Acked-by: David Rientjes <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>