linux-kernel - Re: [PATCH] zsmalloc: use actual object size to detect spans

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Message-ID: <og2yztajena3ywyvus6xgc5vlrrecjaynyezygpsrt64kjkqlv@agceajv57eet>
Date: Wed, 7 Jan 2026 09:59:30 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>, Minchan Kim <minchan@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Brian Geffon <bgeffon@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH] zsmalloc: use actual object size to detect spans

On (26/01/07 00:23), Yosry Ahmed wrote:
> Instead of modifying mem_len, can we modify 'off' like zs_obj_write()
> and zs_obj_read_end()? I think this can actually be done as a prequel to
> this patch. Arguably, it makes more sense as we avoid unnecessarily
> copying the handle (completely untested):
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 5bf832f9c05c..48c288da43b8 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1087,6 +1087,9 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
>         class = zspage_class(pool, zspage);
>         off = offset_in_page(class->size * obj_idx);
> 
> +       if (!ZsHugePage(zspage))
> +               off += ZS_HANDLE_SIZE;
> +
>         if (off + class->size <= PAGE_SIZE) {
>                 /* this object is contained entirely within a page */
>                 addr = kmap_local_zpdesc(zpdesc);
> @@ -1107,9 +1110,6 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
>                                  0, sizes[1]);
>         }
> 
> -       if (!ZsHugePage(zspage))
> -               addr += ZS_HANDLE_SIZE;
> -
>         return addr;
>  }
>  EXPORT_SYMBOL_GPL(zs_obj_read_begin);
> @@ -1129,9 +1129,10 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
>         class = zspage_class(pool, zspage);
>         off = offset_in_page(class->size * obj_idx);
> 
> +       if (!ZsHugePage(zspage))
> +               off += ZS_HANDLE_SIZE;
> +
>         if (off + class->size <= PAGE_SIZE) {
> -               if (!ZsHugePage(zspage))
> -                       off += ZS_HANDLE_SIZE;
>                 handle_mem -= off;
>                 kunmap_local(handle_mem);
>         }
> 
> ---
> Does this work?

Sounds interesting.  Let me try it out.

> > +
> > +	if (off + mem_len <= PAGE_SIZE) {
> >  		/* this object is contained entirely within a page */
> >  		addr = kmap_local_zpdesc(zpdesc);
> >  		addr += off;
> 
> In the else case below (spanning object), should we also use mem_len
> instead of class->size to determine the copy size?

Good catch.

> > @@ -1115,7 +1119,7 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
> >  EXPORT_SYMBOL_GPL(zs_obj_read_begin);
> >  
> >  void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
> > -		     void *handle_mem)
> > +		     size_t mem_len, void *handle_mem)
> >  {
> >  	struct zspage *zspage;
> >  	struct zpdesc *zpdesc;
> > @@ -1129,7 +1133,11 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
> >  	class = zspage_class(pool, zspage);
> >  	off = offset_in_page(class->size * obj_idx);
> >  
> > -	if (off + class->size <= PAGE_SIZE) {
> > +	/* Normal classes have inlined handle */
> > +	if (!ZsHugePage(zspage))
> > +		mem_len += ZS_HANDLE_SIZE;
> > +
> > +	if (off + mem_len <= PAGE_SIZE) {
> >  		if (!ZsHugePage(zspage))
> >  			off += ZS_HANDLE_SIZE;
> 
> With the proposed prequel patch, I think we won't need to handle
> ZS_HANDLE_SIZE twice here. WDYT? Did I miss sth?

Let me look more closely.