zsmalloc: account for handle size in class lookup

zs_lookup_class_index() lets zram recompression decide whether a newly
compressed object would use a smaller size class.  It currently classifies
the payload size directly, while zs_malloc() adds ZS_HANDLE_SIZE before
selecting the class.

This makes lookup disagree with allocation near size-class boundaries. 
With 4 KiB pages, CONFIG_ZSMALLOC_CHAIN_SIZE=8, and 64-bit handles, a
1025-to-1024-byte recompression appears to move from class 64 to class 62
although both allocations use class 64.  Conversely, a 1049-to-1025-byte
recompression appears to stay in class 64 although the allocations move
from class 65 to class 64.

As a result, zram can accept replacements with no allocation benefit or
reject ones that would save memory, potentially marking the object
incompressible.

Factor size-class selection into lookup_size_class(), account for the
handle there, and use the helper for both lookup and allocation.

Link: https://lore.kernel.org/20260809115518.3791787-1-xialonglong2025@163.com
Fixes: 7c2af309abd2 ("zram: add size class equals check into recompression")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 8204b76f7830..825022a7a328 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -552,6 +552,11 @@ static int get_size_class_index(int size)
 	return min_t(int, ZS_SIZE_CLASSES - 1, idx);
 }
 
+static struct size_class *lookup_size_class(struct zs_pool *pool, size_t size)
+{
+	return pool->size_class[get_size_class_index(size + ZS_HANDLE_SIZE)];
+}
+
 static inline void class_stat_add(struct size_class *class, int type,
 				  unsigned long cnt)
 {
@@ -1117,7 +1122,7 @@ unsigned int zs_lookup_class_index(struct zs_pool *pool, unsigned int size)
 {
 	struct size_class *class;
 
-	class = pool->size_class[get_size_class_index(size)];
+	class = lookup_size_class(pool, size);
 
 	return class->index;
 }
@@ -1407,9 +1412,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp,
 	if (!handle)
 		return (unsigned long)ERR_PTR(-ENOMEM);
 
-	/* extra space in chunk to keep the handle */
-	size += ZS_HANDLE_SIZE;
-	class = pool->size_class[get_size_class_index(size)];
+	class = lookup_size_class(pool, size);
 
 	/* class->lock effectively protects the zpage migration */
 	spin_lock(&class->lock);