Re: [PATCH v2 08/11] zsmalloc: encode 1-based obj_idx so valid obj is never 0

[Yosry Ahmed <yosry@kernel.org>]

On Tue, Nov 10, 2026 at 02:48:13PM +0900, Sergey Senozhatsky wrote:
> The next patch embeds struct zs_handle into zram and zswap entries,
> and a handle is considered allocated when its obj is not 0.  But obj
> can be 0: object 0 of a zspage whose first page is PFN 0 encodes to
> 0, and some platforms do give PFN 0 to the page allocator.
> 
> Store obj_idx + 1 in obj, so that a valid obj is never 0, and adjust
> ZS_MIN_ALLOC_SIZE so that the largest index still fits.  Unlike a
> separate tag bit, this doesn't take any bits from obj.  zsmalloc did
> the same before handles moved to handle_cachep, see
> commit 67296874eb1c ("staging: zsmalloc: Ensure handle is never 0 on
> success").
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  mm/zsmalloc.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 3a6a7253fd01..aa436df0cc4d 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -61,7 +61,8 @@
>   * Object location (<PFN>, <obj_idx>) is encoded as
>   * a single (unsigned long) handle value.
>   *
> - * Note that object index <obj_idx> starts from 0.
> + * Note that object index <obj_idx> starts from 0, and is encoded as
> + * <obj_idx> + 1 so that a valid encoded obj value is never 0.

Well, this may be a stupid question, but why not make obj_idx start at
1?