linux-kernel - [PATCH 12/12] mm, swap: no need to clear the shadow explicitly

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Message-Id: <20260126-swap-table-p3-v1-12-a74155fab9b0@tencent.com>
Date: Mon, 26 Jan 2026 01:57:35 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
 Kemeng Shi <shikemeng@...weicloud.com>, Nhat Pham <nphamcs@...il.com>, 
 Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, 
 Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...nel.org>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-kernel@...r.kernel.org, 
 Chris Li <chrisl@...nel.org>, Kairui Song <kasong@...cent.com>
Subject: [PATCH 12/12] mm, swap: no need to clear the shadow explicitly

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

Since we no longer bypass the swap cache, every swap-in will clear the
swap shadow by inserting the folio into the swap table. The only place
we may seem to need to free the swap shadow is when the swap slots are
freed directly without a folio (swap_put_entries_direct). But with the
swap table, that is not needed either. Freeing a slot in the swap table
will set the table entry to NULL, which erases the shadow just fine.

So just delete all explicit shadow clearing, it's no longer needed.
Also, rearrange the freeing.

Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/swap.h       |  1 -
 mm/swap_state.c | 21 ---------------------
 mm/swapfile.c   |  2 --
 3 files changed, 24 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index 3395c2aa1956..087cef49cf69 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -290,7 +290,6 @@ void __swap_cache_del_folio(struct swap_cluster_info *ci,
 			    struct folio *folio, swp_entry_t entry, void *shadow);
 void __swap_cache_replace_folio(struct swap_cluster_info *ci,
 				struct folio *old, struct folio *new);
-void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents);
 
 void show_swap_cache_info(void);
 void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index c808f0948b10..20c4c2414db3 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -350,27 +350,6 @@ void __swap_cache_replace_folio(struct swap_cluster_info *ci,
 	}
 }
 
-/**
- * __swap_cache_clear_shadow - Clears a set of shadows in the swap cache.
- * @entry: The starting index entry.
- * @nr_ents: How many slots need to be cleared.
- *
- * Context: Caller must ensure the range is valid, all in one single cluster,
- * not occupied by any folio, and lock the cluster.
- */
-void __swap_cache_clear_shadow(swp_entry_t entry, int nr_ents)
-{
-	struct swap_cluster_info *ci = __swap_entry_to_cluster(entry);
-	unsigned int ci_off = swp_cluster_offset(entry), ci_end;
-	unsigned long old;
-
-	ci_end = ci_off + nr_ents;
-	do {
-		old = __swap_table_xchg(ci, ci_off, null_to_swp_tb());
-		WARN_ON_ONCE(swp_tb_is_folio(old) || swp_tb_get_count(old));
-	} while (++ci_off < ci_end);
-}
-
 /*
  * If we are the only user, then try to free up the swap cache.
  *
diff --git a/mm/swapfile.c b/mm/swapfile.c
index f2b5013c7692..82b152d7c4c5 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1287,7 +1287,6 @@ static void swap_range_alloc(struct swap_info_struct *si,
 static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 			    unsigned int nr_entries)
 {
-	unsigned long begin = offset;
 	unsigned long end = offset + nr_entries - 1;
 	void (*swap_slot_free_notify)(struct block_device *, unsigned long);
 	unsigned int i;
@@ -1312,7 +1311,6 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 			swap_slot_free_notify(si->bdev, offset);
 		offset++;
 	}
-	__swap_cache_clear_shadow(swp_entry(si->type, begin), nr_entries);
 
 	/*
 	 * Make sure that try_to_unuse() observes si->inuse_pages reaching 0

-- 
2.52.0