linux-kernel - Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf and ftrace [was Re: PyTimechart]]

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Message-ID: <1273862945.1674.14.camel@laptop>
Date:	Fri, 14 May 2010 20:49:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf
 and ftrace [was Re: PyTimechart]]

On Fri, 2010-05-14 at 14:32 -0400, Mathieu Desnoyers wrote:

> [CCing memory management specialists]

And jet you forgot Jens who wrote it ;-)

> So I have three questions here:
> 
> 1 - could we enforce removal of these pages from the page cache by calling
>     "page_cache_release()" before giving these pages back to the ring buffer ?
> 
> 2 - or maybe is there a page flag we could specify when we allocate them to
>     ask for these pages to never be put in the page cache ? (but they should be
>     still usable as write buffers)
> 
> 3 - is there something more we need to do to grab a reference on the pages
>     before passing them to splice(), so that when we call page_cache_release()
>     they don't get reclaimed ? 

There is no guarantee it is the pagecache they end up in, it could be a
network packet queue, a pipe, or anything that implements .splice_write.

>>From what I understand of splice() is that it assumes it passes
ownership of the page, you're not supposed to touch them again, non of
the above three are feasible.



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