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@xxxxxxxxxxxxxxx
> Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>

Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>