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

[Peter Zijlstra <peterz@infradead.org>]

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.