linux-kernel - [PATCH] mm: khugepaged: fix the incorrect statistics when collapsing large file folios

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Message-Id: <c76305d96d12d030a1a346b50503d148364246d2.1728901391.git.baolin.wang@linux.alibaba.com>
Date: Mon, 14 Oct 2024 18:24:44 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: akpm@...ux-foundation.org,
	hughd@...gle.com
Cc: willy@...radead.org,
	david@...hat.com,
	21cnbao@...il.com,
	ryan.roberts@....com,
	shy828301@...il.com,
	ziy@...dia.com,
	baolin.wang@...ux.alibaba.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] mm: khugepaged: fix the incorrect statistics when collapsing large file folios

Khugepaged already supports collapsing file large folios (including shmem mTHP)
by commit 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse"), and the
control parameters in khugepaged: 'khugepaged_max_ptes_swap' and 'khugepaged_max_ptes_none',
still compare based on PTE granularity to determine whether a file collapse is
needed. However, the statistics for 'present' and 'swap' in hpage_collapse_scan_file()
do not take into account the large folios, which may lead to incorrect judgments
regarding the khugepaged_max_ptes_swap/none parameters, resulting in unnecessary
file collapses.

To fix this issue, take into account the large folios' statistics for 'present'
and 'swap' variables in the hpage_collapse_scan_file().

Fixes: 7de856ffd007 ("mm: khugepaged: support shmem mTHP collapse")
Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
---
 mm/khugepaged.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ba28ba09fe89..6f8d46d107b4 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2267,7 +2267,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 			continue;

 		if (xa_is_value(folio)) {
-			++swap;
+			swap += 1 << xas_get_order(&xas);
 			if (cc->is_khugepaged &&
 			    swap > khugepaged_max_ptes_swap) {
 				result = SCAN_EXCEED_SWAP_PTE;
@@ -2314,7 +2314,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 		 * is just too costly...
 		 */

-		present++;
+		present += folio_nr_pages(folio);

 		if (need_resched()) {
 			xas_pause(&xas);
-- 
2.39.3