linux-kernel - [PATCH v2 1/9] mm/swapfile.c: add back some comment

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Date: Wed,  3 Jan 2024 01:53:30 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Chris Li <chrisl@...nel.org>,
	"Huang, Ying" <ying.huang@...el.com>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Matthew Wilcox <willy@...radead.org>,
	Michal Hocko <mhocko@...e.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	David Hildenbrand <david@...hat.com>,
	linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>
Subject: [PATCH v2 1/9] mm/swapfile.c: add back some comment

From: Kairui Song <kasong@...cent.com>

Some useful comments were dropped in commit b56a2d8af914 ('mm: rid
swapoff of quadratic complexity'), add them back.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swapfile.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3eec686484ef..f7271504aa0a 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1880,6 +1880,17 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				folio = page_folio(page);
 		}
 		if (!folio) {
+			/*
+			 * The entry could have been freed, and will not
+			 * be reused since swapoff() already disabled
+			 * allocation from here, or alloc_page() failed.
+			 *
+			 * We don't hold lock here, so the swap entry could be
+			 * SWAP_MAP_BAD (when the cluster is discarding).
+			 * Instead of fail out, We can just skip the swap
+			 * entry because swapoff will wait for discarding
+			 * finish anyway.
+			 */
 			swp_count = READ_ONCE(si->swap_map[offset]);
 			if (swp_count == 0 || swp_count == SWAP_MAP_BAD)
 				continue;
-- 
2.43.0