linux-kernel - [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

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Message-ID: <20241026033625.2237102-1-yuzhao@google.com>
Date: Fri, 25 Oct 2024 21:36:25 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Rientjes <rientjes@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>, 
	Link Lin <linkl@...gle.com>
Subject: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

OOM kills due to vastly overestimated free highatomic reserves were
observed:

  ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ...
  Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ...
  Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB

The second line above shows that the OOM kill was due to the following
condition:

  free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB)

And the third line shows there were no free pages in any
MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type
'H'. Therefore __zone_watermark_unusable_free() underestimated the
usable free memory by over 1GB, which resulted in the unnecessary OOM
kill above.

The comments in __zone_watermark_unusable_free() warns about the
potential risk, i.e.,

  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.

However, it is possible to keep track of free pages in reserved
highatomic pageblocks with a new per-zone counter nr_free_highatomic
protected by the zone lock, to avoid a search when calculating the
usable free memory. And the cost would be minimal, i.e., simple
arithmetics in the highatomic alloc/free/move paths.

Reported-by: Link Lin <linkl@...gle.com>
Signed-off-by: Yu Zhao <yuzhao@...gle.com>
---
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        | 23 ++++++++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 2e8c4307c728..5e8f567753bd 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -825,6 +825,7 @@ struct zone {
 	unsigned long watermark_boost;
 
 	unsigned long nr_reserved_highatomic;
+	unsigned long nr_free_highatomic;
 
 	/*
 	 * We don't know if the memory that we're going to allocate will be
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c81762c49b00..43ecc7d75a2a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -644,6 +644,18 @@ static inline void account_freepages(struct zone *zone, int nr_pages,
 		__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
 }
 
+static void account_highatomic_freepages(struct zone *zone, unsigned int order,
+					 int old_mt, int new_mt)
+{
+	int delta = 1 << order;
+
+	if (is_migrate_highatomic(old_mt))
+		WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic - delta);
+
+	if (is_migrate_highatomic(new_mt))
+		WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic + delta);
+}
+
 /* Used for pages not on another list */
 static inline void __add_to_free_list(struct page *page, struct zone *zone,
 				      unsigned int order, int migratetype,
@@ -660,6 +672,8 @@ static inline void __add_to_free_list(struct page *page, struct zone *zone,
 	else
 		list_add(&page->buddy_list, &area->free_list[migratetype]);
 	area->nr_free++;
+
+	account_highatomic_freepages(zone, order, -1, migratetype);
 }
 
 /*
@@ -681,6 +695,8 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 
 	account_freepages(zone, -(1 << order), old_mt);
 	account_freepages(zone, 1 << order, new_mt);
+
+	account_highatomic_freepages(zone, order, old_mt, new_mt);
 }
 
 static inline void __del_page_from_free_list(struct page *page, struct zone *zone,
@@ -698,6 +714,8 @@ static inline void __del_page_from_free_list(struct page *page, struct zone *zon
 	__ClearPageBuddy(page);
 	set_page_private(page, 0);
 	zone->free_area[order].nr_free--;
+
+	account_highatomic_freepages(zone, order, migratetype, -1);
 }
 
 static inline void del_page_from_free_list(struct page *page, struct zone *zone,
@@ -3119,11 +3137,10 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
 
 	/*
 	 * 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.
+	 * watermark then subtract the free pages reserved for highatomic.
 	 */
 	if (likely(!(alloc_flags & ALLOC_RESERVES)))
-		unusable_free += z->nr_reserved_highatomic;
+		unusable_free += READ_ONCE(z->nr_free_highatomic);
 
 #ifdef CONFIG_CMA
 	/* If allocation can't use CMA areas don't use free CMA pages */
-- 
2.47.0.163.g1226f6d8fa-goog