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Date: Tue, 18 May 2010 11:16:26 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
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       Christoph Lameter <cl@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
       Jens Axboe <jens.axboe@oracle.com>
Subject: Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perf
	and ftrace [was Re: PyTimechart]]
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* Peter Zijlstra (peterz@infradead.org) wrote:
> On Mon, 2010-05-17 at 18:42 -0400, Mathieu Desnoyers wrote:
> > I'll continue to look into this. One of the things I noticed that that we could
> > possibly use the "steal()" operation to steal the pages back from the page cache
> > to repopulate the ring buffer rather than continuously allocating new pages. If
> > steal() fails for some reasons, then we can fall back on page allocation. I'm
> > not sure it is safe to assume anything about pages being in the page cache
> > though. 
> 
> Also, suppose it was still in the page-cache and still dirty, a steal()
> would then punch a hole in the file.

page_cache_pipe_buf_steal starts by doing a wait_on_page_writeback(page); and
then does a try_to_release_page(page, GFP_KERNEL). Only if that succeeds is the
action of stealing succeeding.

> 
> > Maybe the safest route is to just allocate new pages for now.
> 
> Yes, that seems to be the only sane approach.

Yes, a good start anyway.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com