linux-kernel - Re: [PATCH v3 08/16] zsmalloc: squeeze freelist into page->mapping

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Date:	Tue, 19 Apr 2016 16:42:39 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.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>,
	Rik van Riel <riel@...hat.com>, <rknize@...orola.com>,
	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Sangseok Lee <sangseok.lee@....com>,
	Chan Gyun Jeong <chan.jeong@....com>,
	Al Viro <viro@...IV.linux.org.uk>,
	YiPing Xu <xuyiping@...ilicon.com>
Subject: Re: [PATCH v3 08/16] zsmalloc: squeeze freelist into page->mapping

On Mon, Apr 18, 2016 at 12:56:21AM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (03/30/16 16:12), Minchan Kim wrote:
> [..]
> > +static void objidx_to_page_and_offset(struct size_class *class,
> > +				struct page *first_page,
> > +				unsigned long obj_idx,
> > +				struct page **obj_page,
> > +				unsigned long *offset_in_page)
> >  {
> > -	unsigned long obj;
> > +	int i;
> > +	unsigned long offset;
> > +	struct page *cursor;
> > +	int nr_page;
> >  
> > -	if (!page) {
> > -		VM_BUG_ON(obj_idx);
> > -		return NULL;
> > -	}
> > +	offset = obj_idx * class->size;
> 
> so we already know the `offset' before we call objidx_to_page_and_offset(),
> thus we can drop `struct size_class *class' and `obj_idx', and pass
> `long obj_offset'  (which is `obj_idx * class->size') instead, right?
> 
> we also _may be_ can return `cursor' from the function.
> 
> static struct page *objidx_to_page_and_offset(struct page *first_page,
> 					unsigned long obj_offset,
> 					unsigned long *offset_in_page);
> 
> this can save ~20 instructions, which is not so terrible for a hot path
> like obj_malloc(). what do you think?
> 
> well, seems that `unsigned long *offset_in_page' can be calculated
> outside of this function too, it's basically
> 
> 	*offset_in_page = (obj_idx * class->size) & ~PAGE_MASK;
> 
> so we don't need to supply it to this function, nor modify it there.
> which can save ~40 instructions on my system. does this sound silly?

Sound smart. :)
At least, we will use it in hot path.

Thanks!