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

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Message-Id: <20130217145456.375dab11.akpm@linux-foundation.org>
Date:	Sun, 17 Feb 2013 14:54:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Johannes Weiner <hannes@...xchg.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, 16 Feb 2013 14:53:43 +1030 Rusty Russell <rusty@...tcorp.com.au> 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.

That's good news ;)

We shouldn't add it unless there's damn good reason.

> 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.
>
> So why not just use mincore?

One can use fadvise(POSIX_FADV_DONTNEED) to drop the pages.

Or toss your backup app into a small memcg so it reclaims its own
stuff.  See recent thread "mm: fadvise: Drain all pagevecs if
POSIX_FADV_DONTNEED fails to discard all pages"
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