linux-kernel - Re: [PATCH v2 00/13] MADV

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Date:	Sat, 5 Dec 2015 10:51:58 -0500
From:	Daniel Micay <danielmicay@...il.com>
To:	Pavel Machek <pavel@....cz>, Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Evans <je@...com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	yalin.wang2010@...il.com
Subject: Re: [PATCH v2 00/13] MADV_FREE support

On 05/12/15 06:10 AM, Pavel Machek wrote:
> On Wed 2015-11-04 10:25:54, Minchan Kim wrote:
>> MADV_FREE is on linux-next so long time. The reason was two, I think.
>>
>> 1. MADV_FREE code on reclaim path was really mess.
> 
> Could you explain what MADV_FREE does?
> 
> Comment in code says 'free the page only when there's memory
> pressure'. So I mark my caches MADV_FREE, no memory pressure, I can
> keep using it? And if there's memory pressure, what happens? I get
> zeros? SIGSEGV?

You get zeroes. It's not designed for that use case right now. It's for
malloc implementations to use internally. There would need to be a new
feature like MADV_FREE_UNDO for it to be usable for caches and it may
make more sense for that to be a separate feature entirely, i.e. have a
different flag for marking too (not sure) since it wouldn't need to
worry about whether stuff is touched.


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