linux-kernel - Re: missing madvise functionality

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Date:	Wed, 4 Apr 2007 19:39:07 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: missing madvise functionality

On Wed, 4 Apr 2007, Andrew Morton wrote:
> 
> The treatment is identical to clean swapcache pages, with the sole
> exception that they don't actually consume any swap space - hence the fake
> swapcache entry thing.

I see, sneaking through try_to_unmap's anon PageSwapCache assumptions
as simply as possible - thanks.

(Coincidentally, Andrea pointed to precisely the same issue in the
no PAGE_ZERO thread, when we were toying with writable but clean.)

> One thing which we haven't sorted out with all this stuff: once the
> application has marked an address range (and some pages) as
> whatever-were-going-call-this-feature, how does the application undo
> that change?

By re-referencing the pages.  (Hmm, so an incorrect app which accesses
"free"d areas, will undo it: well, okay, nothing terrible about that.)

> What effect will things like mremap, madvise and mlock have upon
> these pages?

mlock will undo the state in its make_pages_present: I guess that
should happen in or near follow_page's mark_page_accessed.

mremap?  Other madvises?  Nothing much at all: mremap can move
them around, and the madvises do whatever they do - I don't notice
any problem in that direction, but it'll be easier when we have an
implementation to poke at.

Hugh
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