linux-kernel - Re: [PATCH 4/5] vrange: Set affected pages referenced when marking volatile

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Date:	Sun, 23 Mar 2014 17:01:29 -0700
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>, Dave Hansen <dave@...1.net>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Jan Kara <jack@...e.cz>, Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 4/5] vrange: Set affected pages referenced when marking volatile

On Fri, Mar 21, 2014 at 2:17 PM, John Stultz <john.stultz@...aro.org> wrote:
> One issue that some potential users were concerned about, was that
> they wanted to ensure that all the pages from one volatile range
> were purged before we purge pages from a different volatile range.
> This would prevent the case where they have 4 large objects, and
> the system purges one page from each object, casuing all of the
> objects to have to be re-created.
>
> The counter-point to this case, is when an application is using the
> SIGBUS semantics to continue to access pages after they have been
> marked volatile. In that case, the desire was that the most recently
> touched pages be purged last, and only the "cold" pages be purged
> from the specified range.
>
> Instead of adding option flags for the various usage model (at least
> initially), one way of getting a solutoin for both uses would be to
> have the act of marking pages as volatile in effect mark the pages
> as accessed. Since all of the pages in the range would be marked
> together, they would be of the same "age" and would (approximately)
> be purged together. Further, if any pages in the range were accessed
> after being marked volatile, they would be moved to the end of the
> lru and be purged later.

If you run after two hares, you will catch neither. I suspect this patch
doesn't make happy any user.
I suggest to aim former case (object level caching) and aim latter by
another patch-kit.
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