Re: [PATCH V4 00/10] mm: page_alloc: freelist migratetype hygiene

[Vlastimil Babka <vbabka@suse.cz>]

On 3/20/24 7:02 PM, Johannes Weiner wrote:
> V4:
> - fixed !pcp_order_allowed() case in free_unref_folios()
> - reworded the patch 0 changelog a bit for the git log
> - rebased to mm-everything-2024-03-19-23-01
> - runtime-tested again with various CONFIG_DEBUG_FOOs enabled
> 
> ---
> 
> The page allocator's mobility grouping is intended to keep unmovable
> pages separate from reclaimable/compactable ones to allow on-demand
> defragmentation for higher-order allocations and huge pages.
> 
> Currently, there are several places where accidental type mixing
> occurs: an allocation asks for a page of a certain migratetype and
> receives another. This ruins pageblocks for compaction, which in turn
> makes allocating huge pages more expensive and less reliable.
> 
> The series addresses those causes. The last patch adds type checks on
> all freelist movements to prevent new violations being introduced.
> 
> The benefits can be seen in a mixed workload that stresses the machine
> with a memcache-type workload and a kernel build job while
> periodically attempting to allocate batches of THP. The following data
> is aggregated over 50 consecutive defconfig builds:

Great stuff. What would you say to the following on top?

----8<----
From 84f8a6d3a9e34c7ed8b438c3152d56e359a4ffb4 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Wed, 27 Mar 2024 10:19:47 +0100
Subject: [PATCH] mm: page_alloc: change move_freepages() to
 __move_freepages_block()

The function is now supposed to be called only on a single pageblock and
checks start_pfn and end_pfn accordingly. Rename it to make this more
obvious and drop the end_pfn parameter which can be determined trivially
and none of the callers use it for anything else.

Also make the (now internal) end_pfn exclusive, which is more common.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/page_alloc.c | 43 ++++++++++++++++++++-----------------------
 1 file changed, 20 insertions(+), 23 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 34c84ef16b66..75aefbd52ef9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1566,18 +1566,18 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
  * Change the type of a block and move all its free pages to that
  * type's freelist.
  */
-static int move_freepages(struct zone *zone, unsigned long start_pfn,
-			  unsigned long end_pfn, int old_mt, int new_mt)
+static int __move_freepages_block(struct zone *zone, unsigned long start_pfn,
+				  int old_mt, int new_mt)
 {
 	struct page *page;
-	unsigned long pfn;
+	unsigned long pfn, end_pfn;
 	unsigned int order;
 	int pages_moved = 0;
 
 	VM_WARN_ON(start_pfn & (pageblock_nr_pages - 1));
-	VM_WARN_ON(start_pfn + pageblock_nr_pages - 1 != end_pfn);
+	end_pfn = pageblock_end_pfn(start_pfn);
 
-	for (pfn = start_pfn; pfn <= end_pfn;) {
+	for (pfn = start_pfn; pfn < end_pfn;) {
 		page = pfn_to_page(pfn);
 		if (!PageBuddy(page)) {
 			pfn++;
@@ -1603,14 +1603,13 @@ static int move_freepages(struct zone *zone, unsigned long start_pfn,
 
 static bool prep_move_freepages_block(struct zone *zone, struct page *page,
 				      unsigned long *start_pfn,
-				      unsigned long *end_pfn,
 				      int *num_free, int *num_movable)
 {
 	unsigned long pfn, start, end;
 
 	pfn = page_to_pfn(page);
 	start = pageblock_start_pfn(pfn);
-	end = pageblock_end_pfn(pfn) - 1;
+	end = pageblock_end_pfn(pfn);
 
 	/*
 	 * The caller only has the lock for @zone, don't touch ranges
@@ -1621,16 +1620,15 @@ static bool prep_move_freepages_block(struct zone *zone, struct page *page,
 	 */
 	if (!zone_spans_pfn(zone, start))
 		return false;
-	if (!zone_spans_pfn(zone, end))
+	if (!zone_spans_pfn(zone, end - 1))
 		return false;
 
 	*start_pfn = start;
-	*end_pfn = end;
 
 	if (num_free) {
 		*num_free = 0;
 		*num_movable = 0;
-		for (pfn = start; pfn <= end;) {
+		for (pfn = start; pfn < end;) {
 			page = pfn_to_page(pfn);
 			if (PageBuddy(page)) {
 				int nr = 1 << buddy_order(page);
@@ -1656,13 +1654,12 @@ static bool prep_move_freepages_block(struct zone *zone, struct page *page,
 static int move_freepages_block(struct zone *zone, struct page *page,
 				int old_mt, int new_mt)
 {
-	unsigned long start_pfn, end_pfn;
+	unsigned long start_pfn;
 
-	if (!prep_move_freepages_block(zone, page, &start_pfn, &end_pfn,
-				       NULL, NULL))
+	if (!prep_move_freepages_block(zone, page, &start_pfn, NULL, NULL))
 		return -1;
 
-	return move_freepages(zone, start_pfn, end_pfn, old_mt, new_mt);
+	return __move_freepages_block(zone, start_pfn, old_mt, new_mt);
 }
 
 #ifdef CONFIG_MEMORY_ISOLATION
@@ -1733,10 +1730,9 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 bool move_freepages_block_isolate(struct zone *zone, struct page *page,
 				  int migratetype)
 {
-	unsigned long start_pfn, end_pfn, pfn;
+	unsigned long start_pfn, pfn;
 
-	if (!prep_move_freepages_block(zone, page, &start_pfn, &end_pfn,
-				       NULL, NULL))
+	if (!prep_move_freepages_block(zone, page, &start_pfn, NULL, NULL))
 		return false;
 
 	/* No splits needed if buddies can't span multiple blocks */
@@ -1767,8 +1763,9 @@ bool move_freepages_block_isolate(struct zone *zone, struct page *page,
 		return true;
 	}
 move:
-	move_freepages(zone, start_pfn, end_pfn,
-		       get_pfnblock_migratetype(page, start_pfn), migratetype);
+	__move_freepages_block(zone, start_pfn,
+			       get_pfnblock_migratetype(page, start_pfn),
+			       migratetype);
 	return true;
 }
 #endif /* CONFIG_MEMORY_ISOLATION */
@@ -1868,7 +1865,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
 			unsigned int alloc_flags, bool whole_block)
 {
 	int free_pages, movable_pages, alike_pages;
-	unsigned long start_pfn, end_pfn;
+	unsigned long start_pfn;
 	int block_type;
 
 	block_type = get_pageblock_migratetype(page);
@@ -1901,8 +1898,8 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
 		goto single_page;
 
 	/* moving whole block can fail due to zone boundary conditions */
-	if (!prep_move_freepages_block(zone, page, &start_pfn, &end_pfn,
-				       &free_pages, &movable_pages))
+	if (!prep_move_freepages_block(zone, page, &start_pfn, &free_pages,
+				       &movable_pages))
 		goto single_page;
 
 	/*
@@ -1932,7 +1929,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
 	 */
 	if (free_pages + alike_pages >= (1 << (pageblock_order-1)) ||
 			page_group_by_mobility_disabled) {
-		move_freepages(zone, start_pfn, end_pfn, block_type, start_type);
+		__move_freepages_block(zone, start_pfn, block_type, start_type);
 		return __rmqueue_smallest(zone, order, start_type);
 	}
 
-- 
2.44.0