linux-kernel - Re: [PATCH v3 01/16] mm: use put_page to free page instead of putback_lru

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Message-ID: <57032CAE.10404@gmail.com>
Date:	Tue, 5 Apr 2016 13:10:38 +1000
From:	Balbir Singh <bsingharora@...il.com>
To:	Minchan Kim <minchan@...nel.org>
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>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.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>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH v3 01/16] mm: use put_page to free page instead of
 putback_lru_page



On 04/04/16 16:01, Minchan Kim wrote:
> On Mon, Apr 04, 2016 at 03:53:59PM +1000, Balbir Singh wrote:
>>
>> On 30/03/16 18:12, Minchan Kim wrote:
>>> Procedure of page migration is as follows:
>>>
>>> First of all, it should isolate a page from LRU and try to
>>> migrate the page. If it is successful, it releases the page
>>> for freeing. Otherwise, it should put the page back to LRU
>>> list.
>>>
>>> For LRU pages, we have used putback_lru_page for both freeing
>>> and putback to LRU list. It's okay because put_page is aware of
>>> LRU list so if it releases last refcount of the page, it removes
>>> the page from LRU list. However, It makes unnecessary operations
>>> (e.g., lru_cache_add, pagevec and flags operations. It would be
>>> not significant but no worth to do) and harder to support new
>>> non-lru page migration because put_page isn't aware of non-lru
>>> page's data structure.
>>>
>>> To solve the problem, we can add new hook in put_page with
>>> PageMovable flags check but it can increase overhead in
>>> hot path and needs new locking scheme to stabilize the flag check
>>> with put_page.
>>>
>>> So, this patch cleans it up to divide two semantic(ie, put and putback).
>>> If migration is successful, use put_page instead of putback_lru_page and
>>> use putback_lru_page only on failure. That makes code more readable
>>> and doesn't add overhead in put_page.
>> So effectively when we return from unmap_and_move() the page is either
>> put_page or putback_lru_page() and the page is gone from under us.
> I didn't get your point.
> Could you elaborate it more what you want to say about this patch?

I was just adding to my understanding of this change based on your changelog.
My understanding is that we take the extra reference in isolate_lru_page()
but by the time we return from unmap_and_move() we drop the extra reference

Balbir Singh