Re: [RFC PATCH 5/6] zsmalloc: introduce handle mapping API

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Mon, Jan 27, 2025 at 04:59:30PM +0900, Sergey Senozhatsky wrote:
> Introduce new API to map/unmap zsmalloc handle/object.  The key
> difference is that this API does not impose atomicity restrictions
> on its users, unlike zs_map_object() which returns with page-faults
> and preemption disabled

I think that's not entirely accurate, see below.

[..]
> @@ -1309,12 +1297,14 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>  		goto out;
>  	}
>  
> -	/* this object spans two pages */
> -	zpdescs[0] = zpdesc;
> -	zpdescs[1] = get_next_zpdesc(zpdesc);
> -	BUG_ON(!zpdescs[1]);
> +	ret = area->vm_buf;
> +	/* disable page faults to match kmap_local_page() return conditions */
> +	pagefault_disable();

Is this accurate/necessary? I am looking at kmap_local_page() and I
don't see it. Maybe that's remnant from the old code using
kmap_atomic()?

> +	if (mm != ZS_MM_WO) {
> +		/* this object spans two pages */
> +		zs_obj_copyin(area->vm_buf, zpdesc, off, class->size);
> +	}
>  
> -	ret = __zs_map_object(area, zpdescs, off, class->size);
>  out:
>  	if (likely(!ZsHugePage(zspage)))
>  		ret += ZS_HANDLE_SIZE;