mm/page_alloc: explicitly define what alloc flags deplete min reserves

As there are more ALLOC_ flags that affect reserves, define what flags
affect reserves and clarify the effect of each flag.

Link: https://lkml.kernel.org/r/20230113111217.14134-5-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/internal.h

@@ -792,6 +792,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
 #define ALLOC_HIGHATOMIC	0x200 /* Allows access to MIGRATE_HIGHATOMIC */
 #define ALLOC_KSWAPD		0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
 
+/* Flags that allow allocations below the min watermark. */
+#define ALLOC_RESERVES (ALLOC_HARDER|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
+
 enum ttu_flags;
 struct tlbflush_unmap_batch;
 

mm/page_alloc.c

@@ -3967,15 +3967,14 @@ ALLOW_ERROR_INJECTION(should_fail_alloc_page, TRUE);
 static inline long __zone_watermark_unusable_free(struct zone *z,
 				unsigned int order, unsigned int alloc_flags)
 {
-	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
 	long unusable_free = (1 << order) - 1;
 
 	/*
-	 * If the caller does not have rights to ALLOC_HARDER then subtract
-	 * the high-atomic reserves. This will over-estimate the size of the
-	 * atomic reserve but it avoids a search.
+	 * 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.
 	 */
-	if (likely(!alloc_harder))
+	if (likely(!(alloc_flags & ALLOC_RESERVES)))
 		unusable_free += z->nr_reserved_highatomic;
 
 #ifdef CONFIG_CMA
@@ -3999,25 +3998,36 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 {
 	long min = mark;
 	int o;
-	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
 
 	/* free_pages may go negative - that's OK */
 	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
 
-	if (alloc_flags & ALLOC_MIN_RESERVE)
-		min -= min / 2;
-
-	if (unlikely(alloc_harder)) {
+	if (unlikely(alloc_flags & ALLOC_RESERVES)) {
 		/*
-		 * OOM victims can try even harder than normal ALLOC_HARDER
+		 * __GFP_HIGH allows access to 50% of the min reserve as well
+		 * as OOM.
+		 */
+		if (alloc_flags & ALLOC_MIN_RESERVE)
+			min -= min / 2;
+
+		/*
+		 * Non-blocking allocations can access some of the reserve
+		 * with more access if also __GFP_HIGH. The reasoning is that
+		 * a non-blocking caller may incur a more severe penalty
+		 * if it cannot get memory quickly, particularly if it's
+		 * also __GFP_HIGH.
+		 */
+		if (alloc_flags & ALLOC_HARDER)
+			min -= min / 4;
+
+		/*
+		 * OOM victims can try even harder than the normal reserve
 		 * users on the grounds that it's definitely going to be in
 		 * the exit path shortly and free memory. Any allocation it
 		 * makes during the free path will be small and short-lived.
 		 */
 		if (alloc_flags & ALLOC_OOM)
 			min -= min / 2;
-		else
-			min -= min / 4;
 	}
 
 	/*