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From:   Kairui Song <ryncsn@gmail.com>
To:     linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        Kairui Song <kasong@tencent.com>
Subject: [PATCH 02/24] mm/swapfile.c: add back some comment
Date:   Mon, 20 Nov 2023 03:47:18 +0800
Message-ID: <20231119194740.94101-3-ryncsn@gmail.com>
In-Reply-To: <20231119194740.94101-1-ryncsn@gmail.com>
References: <20231119194740.94101-1-ryncsn@gmail.com>
Reply-To: Kairui Song <kasong@tencent.com>
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From: Kairui Song <kasong@tencent.com>

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

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

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4bc70f459164..756104ebd585 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1879,6 +1879,17 @@ 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;
-- 
2.42.0