linux-kernel - Re: [PATCH v3 0/2] fadvise: support POSIX_FADV

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Date:	Wed, 13 Jul 2011 00:22:27 +0200
From:	Andrea Righi <andrea@...terlinux.com>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE

On Tue, Jul 12, 2011 at 02:52:49PM -0700, Patrick J. LoPresti wrote:
> 
> > IIUC, NOREUSE means "the application will use this range of the file
> > once". It's something that we do _before_ accessing the file.  And the
> > kernel needs to remember the ranges of NOREUSE data for each file, so
> > that page cache can be immediately dropped after the data has been
> > accessed (if possible).
> 
> I am no expert on the Linux page cache, but my applications have a great
> interest in exercising some control over it...
> 
> Could NOREUSE be as simple as setting a bit on the page that means
> "never mark this page active"?
> 
> Or more conservatively, "clear this bit before marking the page active"?
> 
> So POSIX_FADV_NOREUSE would set the bit on the page.  Then any operation
> that would normally mark the page active would instead merely clear the
> bit.  This would keep the page on the inactive list _after_ the first
> read and allow it to be reclaimed, which is at least in the "spirit" of
> NOREUSE.

If the file data is not in memory you can't set a bit in any struct
page. You could even open a file and execute POSIX_FADV_NOREUSE without
reading any page from the file.

I think it would be better to maintain a list of file offset/length
structures per file descriptor or (as a starting solution) even mark the
entire file as non-cacheable without considering the ranges.

-Andrea
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