[PATCH 1/1] zsmalloc: account for handle size in class lookup

[PATCH 1/1] zsmalloc: account for handle size in class lookup

[Longlong Xia]

From: Longlong Xia <xialonglong@kylinos.cn>

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.

Fixes: 7c2af309abd2 ("zram: add size class equals check into recompression")
Assisted-by: Codex:gpt-5.6-sol
Cc: stable@vger.kernel.org
Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>
---
 mm/zsmalloc.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 83f5820c45f9..41b03a9ecec2 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -478,6 +478,11 @@ 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)
 {
@@ -1021,7 +1026,7 @@ 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;
 }
@@ -1311,9 +1316,7 @@ 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);
-- 
2.43.0

Re: [PATCH 1/1] zsmalloc: account for handle size in class lookup

[Sergey Senozhatsky]

On (26/08/09 19:55), Longlong Xia wrote:
> 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.
> 
> Fixes: 7c2af309abd2 ("zram: add size class equals check into recompression")
> Assisted-by: Codex:gpt-5.6-sol
> Cc: stable@vger.kernel.org
> Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>