From: Barry Song <v-songbaohua@oppo.com>
Date: Fri, 2 Aug 2024 05:52:43 +0000 (+1200)
Subject: mm: clarify swap_count_continued and improve readability for __swap_duplicate
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=6c639cb9b2c08d79380e1a5fc4a00e5a4d43b9d3;p=users%2Fjedix%2Flinux-maple.git

mm: clarify swap_count_continued and improve readability for __swap_duplicate

When usage=1 and swapcount is very large, the situation can become quite
complex.  The first entry might succeed with swap_count_continued(), but
the second entry may require extending to an additional continued page.
Rolling back these changes can be extremely challenging.  Therefore,
anyone using usage==1 for batched __swap_duplicate() operations should
proceed with caution.

Additionally, we have moved swap_count_continued() to the second iteration
to enhance readability, as this function may modify data.

Link: https://lkml.kernel.org/r/20240802071817.47081-1-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Suggested-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 757d38a86f568..0259f36de715f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3386,6 +3386,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 
 	offset = swp_offset(entry);
 	VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
+	VM_WARN_ON(usage == 1 && nr > 1);
 	ci = lock_cluster_or_swap_info(p, offset);
 
 	err = 0;
@@ -3404,27 +3405,14 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 		has_cache = count & SWAP_HAS_CACHE;
 		count &= ~SWAP_HAS_CACHE;
 
-		if (usage == SWAP_HAS_CACHE) {
-			/* set SWAP_HAS_CACHE if there is no cache and entry is used */
-			if (!has_cache && count)
-				continue;
-			else if (has_cache)		/* someone else added cache */
+		if (!count && !has_cache) {
+			err = -ENOENT;
+		} else if (usage == SWAP_HAS_CACHE) {
+			if (has_cache)
 				err = -EEXIST;
-			else				/* no users remaining */
-				err = -ENOENT;
-
-		} else if (count || has_cache) {
-
-			if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
-				continue;
-			else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
-				err = -EINVAL;
-			else if (swap_count_continued(p, offset + i, count))
-				continue;
-			else
-				err = -ENOMEM;
-		} else
-			err = -ENOENT;			/* unused swap entry */
+		} else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
+			err = -EINVAL;
+		}
 
 		if (err)
 			goto unlock_out;
@@ -3439,8 +3427,16 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
 			has_cache = SWAP_HAS_CACHE;
 		else if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
 			count += usage;
-		else
+		else if (swap_count_continued(p, offset + i, count))
 			count = COUNT_CONTINUED;
+		else {
+			/*
+			 * Don't need to rollback changes, because if
+			 * usage == 1, there must be nr == 1.
+			 */
+			err = -ENOMEM;
+			goto unlock_out;
+		}
 
 		WRITE_ONCE(p->swap_map[offset + i], count | has_cache);
 	}