linux-kernel - [PATCH 2/2] mm,thp,compaction,cma: allow THP migration for CMA allocations

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Date:   Fri, 21 Feb 2020 16:53:43 -0500
From:   Rik van Riel <riel@...riel.com>
To:     linux-kernel@...r.kernel.org, riel@...com
Cc:     kernel-team@...com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        mhocko@...nel.org, vbabka@...e.cz, mgorman@...hsingularity.net,
        rientjes@...gle.com, aarcange@...hat.com,
        Rik van Riel <riel@...riel.com>
Subject: [PATCH 2/2] mm,thp,compaction,cma: allow THP migration for CMA allocations

The code to implement THP migrations already exists, and the code
for CMA to clear out a region of memory already exists.

Only a few small tweaks are needed to allow CMA to move THP memory
when attempting an allocation from alloc_contig_range.

With these changes, migrating THPs from a CMA area works when
allocating a 1GB hugepage from CMA memory.

Signed-off-by: Rik van Riel <riel@...riel.com>
---
 mm/compaction.c | 16 +++++++++-------
 mm/page_alloc.c |  6 ++++--
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 672d3c78c6ab..f3e05c91df62 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -894,12 +894,12 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 
 		/*
 		 * Regardless of being on LRU, compound pages such as THP and
-		 * hugetlbfs are not to be compacted. We can potentially save
-		 * a lot of iterations if we skip them at once. The check is
-		 * racy, but we can consider only valid values and the only
-		 * danger is skipping too much.
+		 * hugetlbfs are not to be compacted most of the time. We can
+		 * potentially save a lot of iterations if we skip them at
+		 * once. The check is racy, but we can consider only valid
+		 * values and the only danger is skipping too much.
 		 */
-		if (PageCompound(page)) {
+		if (PageCompound(page) && !cc->alloc_contig) {
 			const unsigned int order = compound_order(page);
 
 			if (likely(order < MAX_ORDER))
@@ -969,7 +969,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 			 * and it's on LRU. It can only be a THP so the order
 			 * is safe to read and it's 0 for tail pages.
 			 */
-			if (unlikely(PageCompound(page))) {
+			if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
 				low_pfn += compound_nr(page) - 1;
 				goto isolate_fail;
 			}
@@ -981,7 +981,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (__isolate_lru_page(page, isolate_mode) != 0)
 			goto isolate_fail;
 
-		VM_BUG_ON_PAGE(PageCompound(page), page);
+		/* The whole page is taken off the LRU; skip the tail pages. */
+		if (PageCompound(page))
+			low_pfn += compound_nr(page) - 1;
 
 		/* Successfully isolated */
 		del_page_from_lru_list(page, lruvec, page_lru(page));
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a36736812596..38c8ddfcecc8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8253,14 +8253,16 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
 
 		/*
 		 * Hugepages are not in LRU lists, but they're movable.
+		 * THPs are on the LRU, but need to be counted as #small pages.
 		 * We need not scan over tail pages because we don't
 		 * handle each tail page individually in migration.
 		 */
-		if (PageHuge(page)) {
+		if (PageTransHuge(page)) {
 			struct page *head = compound_head(page);
 			unsigned int skip_pages;
 
-			if (!hugepage_migration_supported(page_hstate(head)))
+			if (PageHuge(page) &&
+			    !hugepage_migration_supported(page_hstate(head)))
 				return page;
 
 			skip_pages = compound_nr(head) - (page - head);
-- 
2.24.1