linux-kernel - [PATCH] mm/zswap: remove SWP_SYNCHRONOUS

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Message-Id: <20260202-zswap-syncio-cleanup-v1-1-86bb24a64521@tencent.com>
Date: Mon, 02 Feb 2026 01:47:32 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Johannes Weiner <hannes@...xchg.org>, 
 Yosry Ahmed <yosry.ahmed@...ux.dev>, Nhat Pham <nphamcs@...il.com>, 
 Chengming Zhou <chengming.zhou@...ux.dev>, 
 Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, 
 Barry Song <baohua@...nel.org>, Chris Li <chrisl@...nel.org>, 
 linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: [PATCH] mm/zswap: remove SWP_SYNCHRONOUS_IO swapcache bypass
 workaround

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

Since commit f1879e8a0c60 ("mm, swap: never bypass the swap cache even
for SWP_SYNCHRONOUS_IO"), all swap-in operations go through the swap
cache, including those from SWP_SYNCHRONOUS_IO devices like
zram. Which means the workaround for swap cache bypassing
introduced by commit 25cd241408a2 ("mm: zswap: fix data loss on
SWP_SYNCHRONOUS_IO devices") is no longer needed. Remove it, but
keep the comments that are still helpful.

Suggested-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
Signed-off-by: Kairui Song <kasong@...cent.com>
---
 mm/zswap.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 3d2d59ac3f9c..8cd61603ff79 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1589,11 +1589,11 @@ int zswap_load(struct folio *folio)
 {
 	swp_entry_t swp = folio->swap;
 	pgoff_t offset = swp_offset(swp);
-	bool swapcache = folio_test_swapcache(folio);
 	struct xarray *tree = swap_zswap_tree(swp);
 	struct zswap_entry *entry;
 
 	VM_WARN_ON_ONCE(!folio_test_locked(folio));
+	VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
 
 	if (zswap_never_enabled())
 		return -ENOENT;
@@ -1624,22 +1624,15 @@ int zswap_load(struct folio *folio)
 		count_objcg_events(entry->objcg, ZSWPIN, 1);
 
 	/*
-	 * When reading into the swapcache, invalidate our entry. The
-	 * swapcache can be the authoritative owner of the page and
+	 * We are reading into the swapcache, invalidate zswap entry.
+	 * The swapcache is the authoritative owner of the page and
 	 * its mappings, and the pressure that results from having two
 	 * in-memory copies outweighs any benefits of caching the
 	 * compression work.
-	 *
-	 * (Most swapins go through the swapcache. The notable
-	 * exception is the singleton fault on SWP_SYNCHRONOUS_IO
-	 * files, which reads into a private page and may free it if
-	 * the fault fails. We remain the primary owner of the entry.)
 	 */
-	if (swapcache) {
-		folio_mark_dirty(folio);
-		xa_erase(tree, offset);
-		zswap_entry_free(entry);
-	}
+	folio_mark_dirty(folio);
+	xa_erase(tree, offset);
+	zswap_entry_free(entry);
 
 	folio_unlock(folio);
 	return 0;

---
base-commit: 2c263046cbe6d9d5fce3dfeba063f199f7e6298f
change-id: 20251226-zswap-syncio-cleanup-a05b7fc6180f

Best regards,
-- 
Kairui Song <kasong@...cent.com>