mm/page_alloc: ignore the exact initial compaction result

mm/page_alloc: ignore the exact initial compaction result

Patch series "tweaks for __alloc_pages_slowpath()", v3.

This patch (of 3):

For allocations that are of costly order and __GFP_NORETRY (and can
perform compaction) we attempt direct compaction first.  If that fails, we
continue with a single round of direct reclaim+compaction (as for other
__GFP_NORETRY allocations, except the compaction is of lower priority),
with two exceptions that fail immediately:

- __GFP_THISNODE is specified, to prevent zone_reclaim_mode-like
  behavior for e.g. THP page faults

- compaction failed because it was deferred (i.e. has been failing
  recently so further attempts are not done for a while) or skipped,
  which means there are insufficient free base pages to defragment to
  begin with

Upon closer inspection, the second condition has a somewhat flawed
reasoning.  If there are not enough base pages and reclaim could create
them, we instead fail.  When there are enough base pages and compaction
has already ran and failed, we proceed and hope that reclaim and the
subsequent compaction attempt will succeed.  But it's unclear why they
should and whether it will be as inexpensive as intended.

It might make therefore more sense to just fail unconditionally after the
initial compaction attempt.  However that would change the semantics of
__GFP_NORETRY to attempt reclaim at least once.

Alternatively we can remove the compaction result checks and proceed with
the single reclaim and (lower priority) compaction attempt, leaving only
the __GFP_THISNODE exception for failing immediately.

Link: https://lkml.kernel.org/r/20260106-thp-thisnode-tweak-v3-0-f5d67c21a193@suse.cz
Link: https://lkml.kernel.org/r/20260106-thp-thisnode-tweak-v3-1-f5d67c21a193@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>