mm: use zone lock guard in reserve_highatomic_pageblock()

mm: use zone lock guard in reserve_highatomic_pageblock()

Patch series "mm: use spinlock guards for zone lock", v3.

This series uses spinlock guard for zone lock across several mm functions
to replace explicit lock/unlock patterns with automatic scope-based
cleanup.

This simplifies the control flow by removing 'flags' variables, goto
labels, and redundant unlock calls.

Patches are ordered by decreasing value.  The first six patches simplify
the control flow by removing gotos, multiple unlock paths, or 'ret'
variables.  The last two are simpler lock/unlock pair conversions that
only remove 'flags' and can be dropped if considered unnecessary churn.

Binary size increase is +39 bytes, with Peter Zijlstra's fix for guards
[1] applied.  This is due to the compiler not being able to deduplicate
epilogue and eliminate redundant NULL check.  See discussion [2] for more
details.  I proposed a patch [3] that fixes this, but until it is merged
we need to assume +39 bytes will stay (though it is compiler dependent).

This patch (of 8):

Use the spinlock_irqsave zone lock guard in reserve_highatomic_pageblock()
to replace the explicit lock/unlock and goto out_unlock pattern with
automatic scope-based cleanup.

Link: https://lore.kernel.org/cover.1777462630.git.d@ilvokhin.com
Link: https://lore.kernel.org/3657e1144e2ffc1ca0eb57d57d89bfec4073d8c6.1777462630.git.d@ilvokhin.com
Link: https://lore.kernel.org/all/20260309164516.GE606826@noisy.programming.kicks-ass.net/
Link: https://lore.kernel.org/all/afC5C6fylF4AsITV@shell.ilvokhin.com/
Link: https://lore.kernel.org/all/20260427165037.205337-1-d@ilvokhin.com/
Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>