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>

0 files changed, 0 insertions, 0 deletions