Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating non-CMA THP-sized page

From: Baolin Wang
Date: Wed Jun 05 2024 - 23:06:40 EST




On 2024/6/4 20:36, yangge1116 wrote:


在 2024/6/4 下午8:01, Baolin Wang 写道:
Cc Johannes, Zi and Vlastimil.

On 2024/6/4 17:14, yangge1116@xxxxxxx wrote:
From: yangge <yangge1116@xxxxxxx>

Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for
THP-sized allocations") no longer differentiates the migration type
of pages in THP-sized PCP list, it's possible to get a CMA page from
the list, in some cases, it's not acceptable, for example, allocating
a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page.

The patch forbids allocating non-CMA THP-sized page from THP-sized
PCP list to avoid the issue above.

Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations")
Signed-off-by: yangge <yangge1116@xxxxxxx>
---
  mm/page_alloc.c | 10 ++++++++++
  1 file changed, 10 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2e22ce5..0bdf471 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone *preferred_zone,
      WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
      if (likely(pcp_allowed_order(order))) {
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+        if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
+                        order != HPAGE_PMD_ORDER) {

Seems you will also miss the non-CMA THP from the PCP, so I wonder if we can add a migratetype comparison in __rmqueue_pcplist(), and if it's not suitable, then fallback to buddy?

Yes, we may miss some non-CMA THPs in the PCP. But, if add a migratetype comparison in __rmqueue_pcplist(), we may need to compare many times because of pcp batch.

I mean we can only compare once, focusing on CMA pages.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3734fe7e67c0..960a3b5744d8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2973,6 +2973,11 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
}

page = list_first_entry(list, struct page, pcp_list);
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ if (order == HPAGE_PMD_ORDER && !is_migrate_movable(migratetype) &&
+ is_migrate_cma(get_pageblock_migratetype(page)))
+ return NULL;
+#endif
list_del(&page->pcp_list);
pcp->count -= 1 << order;
} while (check_new_pages(page, order));