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From:   Mel Gorman <mgorman@techsingularity.net>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@suse.com>, NeilBrown <neilb@suse.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: [PATCH 5/6] mm/page_alloc: Explicitly define how __GFP_HIGH non-blocking allocations accesses reserves
Date:   Fri, 13 Jan 2023 11:12:16 +0000
Message-Id: <20230113111217.14134-6-mgorman@techsingularity.net>
In-Reply-To: <20230113111217.14134-1-mgorman@techsingularity.net>
References: <20230113111217.14134-1-mgorman@techsingularity.net>
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GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a vague
description. In preparation for the removal of GFP_ATOMIC redefine
__GFP_ATOMIC to simply mean non-blocking and renaming ALLOC_HARDER to
ALLOC_NON_BLOCK accordingly. __GFP_HIGH is required for access to reserves
but non-blocking is granted more access. For example, GFP_NOWAIT is
non-blocking but has no special access to reserves. A __GFP_NOFAIL
blocking allocation is granted access similar to __GFP_HIGH if the
only alternative is an OOM kill.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/internal.h   |  7 +++++--
 mm/page_alloc.c | 44 ++++++++++++++++++++++++--------------------
 2 files changed, 29 insertions(+), 22 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 8706d46863df..23a37588073a 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -735,7 +735,10 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
 #define ALLOC_OOM		ALLOC_NO_WATERMARKS
 #endif
 
-#define ALLOC_HARDER		 0x10 /* try to alloc harder */
+#define ALLOC_NON_BLOCK		 0x10 /* Caller cannot block. Allow access
+				       * to 25% of the min watermark or
+				       * 62.5% if __GFP_HIGH is set.
+				       */
 #define ALLOC_MIN_RESERVE	 0x20 /* __GFP_HIGH set. Allow access to 50%
 				       * of the min watermark.
 				       */
@@ -750,7 +753,7 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
 #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)
+#define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
 
 enum ttu_flags;
 struct tlbflush_unmap_batch;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6f41b84a97ac..b9ae0ba0a2ab 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3989,18 +3989,19 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 		 * __GFP_HIGH allows access to 50% of the min reserve as well
 		 * as OOM.
 		 */
-		if (alloc_flags & ALLOC_MIN_RESERVE)
+		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;
+			/*
+			 * Non-blocking allocations (e.g. GFP_ATOMIC) can
+			 * access more reserves than just __GFP_HIGH. Other
+			 * non-blocking allocations requests such as GFP_NOWAIT
+			 * or (GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) do not get
+			 * access to the min reserve.
+			 */
+			if (alloc_flags & ALLOC_NON_BLOCK)
+				min -= min / 4;
+		}
 
 		/*
 		 * OOM victims can try even harder than the normal reserve
@@ -4851,28 +4852,30 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
 	 * The caller may dip into page reserves a bit more if the caller
 	 * cannot run direct reclaim, or if the caller has realtime scheduling
 	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
-	 * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_MIN_RESERVE(__GFP_HIGH).
+	 * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH).
 	 */
 	alloc_flags |= (__force int)
 		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
 
-	if (gfp_mask & __GFP_ATOMIC) {
+	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
 		/*
 		 * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
 		 * if it can't schedule.
 		 */
 		if (!(gfp_mask & __GFP_NOMEMALLOC)) {
-			alloc_flags |= ALLOC_HARDER;
+			alloc_flags |= ALLOC_NON_BLOCK;
 
 			if (order > 0)
 				alloc_flags |= ALLOC_HIGHATOMIC;
 		}
 
 		/*
-		 * Ignore cpuset mems for GFP_ATOMIC rather than fail, see the
-		 * comment for __cpuset_node_allowed().
+		 * Ignore cpuset mems for non-blocking __GFP_HIGH (probably
+		 * GFP_ATOMIC) rather than fail, see the comment for
+		 * __cpuset_node_allowed().
 		 */
-		alloc_flags &= ~ALLOC_CPUSET;
+		if (alloc_flags & ALLOC_MIN_RESERVE)
+			alloc_flags &= ~ALLOC_CPUSET;
 	} else if (unlikely(rt_task(current)) && in_task())
 		alloc_flags |= ALLOC_MIN_RESERVE;
 
@@ -5303,12 +5306,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		WARN_ON_ONCE_GFP(costly_order, gfp_mask);
 
 		/*
-		 * Help non-failing allocations by giving them access to memory
-		 * reserves but do not use ALLOC_NO_WATERMARKS because this
+		 * Help non-failing allocations by giving some access to memory
+		 * reserves normally used for high priority non-blocking
+		 * allocations but do not use ALLOC_NO_WATERMARKS because this
 		 * could deplete whole memory reserves which would just make
-		 * the situation worse
+		 * the situation worse.
 		 */
-		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_HARDER, ac);
+		page = __alloc_pages_cpuset_fallback(gfp_mask, order, ALLOC_MIN_RESERVE, ac);
 		if (page)
 			goto got_pg;
 
-- 
2.35.3