linux-kernel - Re: [PATCH v1 11/19] zsmalloc: squeeze freelist into page->mapping

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Date:	Tue, 15 Mar 2016 15:51:26 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, jlayton@...chiereds.net,
	bfields@...ldses.org, Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, koct9i@...il.com,
	aquini@...hat.com, virtualization@...ts.linux-foundation.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	rknize@...orola.com, Rik van Riel <riel@...hat.com>,
	Gioh Kim <gurugio@...mail.net>
Subject: Re: [PATCH v1 11/19] zsmalloc: squeeze freelist into page->mapping

On Tue, Mar 15, 2016 at 03:40:53PM +0900, Sergey Senozhatsky wrote:
> On (03/11/16 16:30), Minchan Kim wrote:
> > -static void *location_to_obj(struct page *page, unsigned long obj_idx)
> > +static void objidx_to_page_and_ofs(struct size_class *class,
> > +				struct page *first_page,
> > +				unsigned long obj_idx,
> > +				struct page **obj_page,
> > +				unsigned long *ofs_in_page)
> 
> this looks big; 5 params, function "returning" both page and offset...
> any chance to split it in two steps, perhaps?

Yes, it's rather ugly but I don't have a good idea.
Feel free to suggest if you have a better idea.

> 
> besides, it is more intuitive (at least to me) when 'offset'
> shortened to 'offt', not 'ofs'.

Indeed. I will change it to get_page_and_offset instead of
abbreviation if we cannot refactor it more.

> 
> 	-ss
> 
> >  {
> > -	unsigned long obj;
> > +	int i;
> > +	unsigned long ofs;
> > +	struct page *cursor;
> > +	int nr_page;
> >  
> > -	if (!page) {
> > -		VM_BUG_ON(obj_idx);
> > -		return NULL;
> > -	}
> > +	ofs = obj_idx * class->size;
> > +	cursor = first_page;
> > +	nr_page = ofs >> PAGE_SHIFT;
> >  
> > -	obj = page_to_pfn(page) << OBJ_INDEX_BITS;
> > -	obj |= ((obj_idx) & OBJ_INDEX_MASK);
> > -	obj <<= OBJ_TAG_BITS;
> > +	*ofs_in_page = ofs & ~PAGE_MASK;
> > +
> > +	for (i = 0; i < nr_page; i++)
> > +		cursor = get_next_page(cursor);
> >  
> > -	return (void *)obj;
> > +	*obj_page = cursor;
> >  }