linux-kernel - Re: [PATCH v1 01/19] mm: use put_page to free page instead of putback_lru

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Date:	Mon, 14 Mar 2016 09:48:33 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	jlayton@...chiereds.net, bfields@...ldses.org,
	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>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH v1 01/19] mm: use put_page to free page instead of
 putback_lru_page

On 03/11/2016 08:30 AM, 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.

Yeah, and compaction (perhaps also other migration users) has to drain 
the lru pvec... Getting rid of this stuff is worth even by itself.

> 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.

I had an idea of checking for count==1 in putback_lru_page() which would 
take the put_page() shortcut from there. But maybe it can't be done 
nicely without races.

> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Note in -next/after 4.6-rc1 this will need some rebasing though.