mm/page_alloc: simplify __alloc_pages_slowpath() flow

mm/page_alloc: simplify __alloc_pages_slowpath() flow

The actions done before entering the main retry loop include waking up
kswapds and an allocation attempt with the precise alloc_flags.  Then in
the loop we keep waking up kswapds, and we retry the allocation with flags
potentially further adjusted by being allowed to use reserves (due to e.g.
becoming an OOM killer victim).

We can adjust the retry loop to keep only one instance of waking up
kswapds and allocation attempt.  Introduce the can_retry_reserves variable
for retrying once when we become eligible for reserves.  It is still
useful not to evaluate reserve_flags immediately for the first allocation
attempt, because it's better to first try succeed in a non-preferred zone
above the min watermark before allocating immediately from the preferred
zone below min watermark.

Additionally move the cpuset update checks introduced by e05741fb10c3
("mm/page_alloc.c: avoid infinite retries caused by cpuset race") further
down the retry loop.  It's enough to do the checks only before reaching
any potentially infinite 'goto retry;' loop.

There should be no meaningful functional changes.  The change of exact
moments the retry for reserves and cpuset updates are checked should not
result in different outomes modulo races with concurrent allocator
activity.

Link: https://lkml.kernel.org/r/20260106-thp-thisnode-tweak-v3-3-f5d67c21a193@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: David Rientjes <rientjes@google.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>