[PATCH] mm: clarify swap_count_continued and improve readability for __swap_duplicate

[Barry Song]

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.

Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
 mm/swapfile.c | 38 +++++++++++++++++---------------------
 1 file changed, 17 insertions(+), 21 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index d9cf31b04db3..ea023fc25d08 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3540,6 +3540,7 @@ 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;
@@ -3558,27 +3559,14 @@ 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;
@@ -3593,8 +3581,16 @@ 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);
 	}
-- 
2.34.1


> >> >> >> >> >> Best Regards,
> >> >> >> >> >> Huang, Ying
> >> >> >> >> >
> >> >> >
> >

Thanks
Barry