[RFC PATCH] mm: Drain PCP during direct reclaim

[Wupeng Ma <mawupeng1@huawei.com>]

Memory retained in Per-CPU Pages (PCP) caches can prevent hugepage
allocations from succeeding despite sufficient free system memory. This
occurs because:
1. Hugepage allocations don't actively trigger PCP draining
2. Direct reclaim path fails to trigger drain_all_pages() when:
   a) All zone pages are free/hugetlb (!did_some_progress)
   b) Compaction skips due to costly order watermarks (COMPACT_SKIPPED)

Reproduction:
  - Alloc page and free the page via put_page to release to pcp
  - Observe hugepage reservation failure

Solution:
  Actively drain PCP during direct reclaim for memory allocations.
  This increases page allocation success rate by making stranded pages
  available to any order allocations.

Verification:
  This issue can be reproduce easily in zone movable with the following
  step:

w/o this patch
  # numactl -m 2 dd if=/dev/urandom of=/dev/shm/testfile bs=4k count=64
  # rm -f /dev/shm/testfile
  # sync
  # echo 3 > /proc/sys/vm/drop_caches
  # echo 2048 > /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
  # cat /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
    2029

w/ this patch
  # numactl -m 2 dd if=/dev/urandom of=/dev/shm/testfile bs=4k count=64
  # rm -f /dev/shm/testfile
  # sync
  # echo 3 > /proc/sys/vm/drop_caches
  # echo 2048 > /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
  # cat /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
    2047

Signed-off-by: Wupeng Ma <mawupeng1@huawei.com>
---
 mm/page_alloc.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ef3c07266b3..464f2e48651e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4137,28 +4137,22 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
 {
 	struct page *page = NULL;
 	unsigned long pflags;
-	bool drained = false;
 
 	psi_memstall_enter(&pflags);
 	*did_some_progress = __perform_reclaim(gfp_mask, order, ac);
-	if (unlikely(!(*did_some_progress)))
-		goto out;
-
-retry:
-	page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
+	if (likely(*did_some_progress))
+		page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
 
 	/*
 	 * If an allocation failed after direct reclaim, it could be because
 	 * pages are pinned on the per-cpu lists or in high alloc reserves.
 	 * Shrink them and try again
 	 */
-	if (!page && !drained) {
+	if (!page) {
 		unreserve_highatomic_pageblock(ac, false);
 		drain_all_pages(NULL);
-		drained = true;
-		goto retry;
+		page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
 	}
-out:
 	psi_memstall_leave(&pflags);
 
 	return page;
-- 
2.43.0