linux-kernel - Re: [PATCH v3 1/2] mm: introduce page reference manipulation functions

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Date:	Tue, 23 Feb 2016 15:32:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	js1304@...il.com
Cc:	Michal Nazarewicz <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 1/2] mm: introduce page reference manipulation
 functions

On Tue, 23 Feb 2016 16:21:17 +0900 js1304@...il.com wrote:

> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Success of CMA allocation largely depends on success of migration
> and key factor of it is page reference count. Until now, page reference
> is manipulated by direct calling atomic functions so we cannot follow up
> who and where manipulate it. Then, it is hard to find actual reason
> of CMA allocation failure. CMA allocation should be guaranteed to succeed
> so finding offending place is really important.
> 
> In this patch, call sites where page reference is manipulated are converted
> to introduced wrapper function. This is preparation step to add tracepoint
> to each page reference manipulation function. With this facility, we can
> easily find reason of CMA allocation failure. There is no functional change
> in this patch.
> 
> ...
>
> --- a/arch/mips/mm/gup.c
> +++ b/arch/mips/mm/gup.c
> @@ -64,7 +64,7 @@ static inline void get_head_page_multiple(struct page *page, int nr)
>  {
>  	VM_BUG_ON(page != compound_head(page));
>  	VM_BUG_ON(page_count(page) == 0);
> -	atomic_add(nr, &page->_count);
> +	page_ref_add(page, nr);

Seems reasonable.  Those open-coded refcount manipulations have always
bugged me.

The patches will be a bit of a pain to maintain but surprisingly they
apply OK at present.  It's possible that by the time they hit upstream,
some direct ->_count references will still be present and it will
require a second pass to complete the conversion.

After that pass is completed I suggest we rename page._count to
something else (page.ref_count_dont_use_this_directly_you_dope?).  That
way, any attempts to later add direct page._count references will
hopefully break, alerting the programmer to the new regime.