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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160315011656.GD19514@bbox>
Date:	Tue, 15 Mar 2016 10:16:56 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, 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 Mon, Mar 14, 2016 at 09:48:33AM +0100, Vlastimil Babka wrote:
> 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.

Good note. Although we cannot remove lru pvec draining completely,
at least, this patch removes a case which should drain pvec for
returning freed page to buddy.

Thanks for the notice.

> 
> >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.

I thought about it and we might do it via page_freeze_refs but
what I want at this moment is to separte two semantic put and putback.
;-)

> 
> >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.

Thanks for the review!