linux-kernel - Re: [patch 1/2] mm: fincore()

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Message-ID: <20130217225112.GB24384@cmpxchg.org>
Date:	Sun, 17 Feb 2013 17:51:12 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Stewart Smith <stewart@...mingspork.com>, linux-mm@...ck.org,
	linux-arch@...r.kernel.org
Subject: Re: [patch 1/2] mm: fincore()

On Sat, Feb 16, 2013 at 02:53:43PM +1030, Rusty Russell wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> > On Fri, 15 Feb 2013 18:13:04 -0500
> > Johannes Weiner <hannes@...xchg.org> wrote:
> >> I dunno.  The byte vector might not be optimal but its worst cases
> >> seem more attractive, is just as extensible, and dead simple to use.
> >
> > But I think "which pages from this 4TB file are in core" will not be an
> > uncommon usage, and writing a gig of memory to find three pages is just
> > awful.
> 
> Actually, I don't know of any usage for this call.
> 
> I'd really like to use it for backup programs, so they stop pulling
> random crap into memory (but leave things already resident).  But that
> needs to madvise(MADV_DONTNEED) on the page, so need mmap.

We do actually have fadvise() (posix_fadvise()).

Btw, why are we not invalidating page cache from MADV_DONTNEED?  I
just see a page table teardown in there, so mmap for madvise alone
won't do any good.  fadvise() OTOTH /does/ invalidate page cache.
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