+ zsmalloc-account-for-handle-size-in-class-lookup.patch added to mm-new branch

The patch titled
     Subject: zsmalloc: account for handle size in class lookup
has been added to the -mm mm-new branch.  Its filename is
     zsmalloc-account-for-handle-size-in-class-lookup.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zsmalloc-account-for-handle-size-in-class-lookup.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
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review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Longlong Xia <[email protected]>
Subject: zsmalloc: account for handle size in class lookup
Date: Sun, 9 Aug 2026 19:55:18 +0800

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/[email protected]
Fixes: 7c2af309abd2 ("zram: add size class equals check into recompression")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Longlong Xia <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/zsmalloc.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/mm/zsmalloc.c~zsmalloc-account-for-handle-size-in-class-lookup
+++ a/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(struc
 {
 	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 *
 	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);
_

Patches currently in -mm which might be from [email protected] are

mm-ksm-avoid-missing-ksmd-wakeups-in-ksm_enter.patch
zram-fix-out-of-bounds-access-in-writeback_store.patch
zram-fix-out-of-bounds-access-in-read_block_state.patch
zsmalloc-account-for-handle-size-in-class-lookup.patch