linux-kernel - Re: [PATCH] Check for compound pages in set_page

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Message-ID: <Pine.LNX.4.64.0707261855290.13876@blonde.wat.veritas.com>
Date:	Thu, 26 Jul 2007 18:59:21 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....com>
cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>,
	David Gibson <david@...son.dropbear.id.au>,
	Ken Chen <kenchen@...gle.com>, Bill Irwin <wli@...omorphy.com>
Subject: Re: [PATCH] Check for compound pages in set_page_dirty()

On Thu, 26 Jul 2007, Christoph Lameter wrote:
> On Thu, 26 Jul 2007, Hugh Dickins wrote:
> 
> > I expect we could take that approach in the current kernel, yes
> > (though it would put those compound tests into the bio code that
> > Jens was preferring to remove).  But I think not if your variable
> > page_cache_size went in: imagine an mmap of the tail component page
> > of an order-1 page_cache_size page, and that pte only being dirtied:
> > wouldn't set_page_dirty on that page need to redirect to the head?
> 
> We would need to redirect all of the page state determinations and changes 
> to the head page anyways. So the memory.c code would have to deal with two 
> struct page pointers: One to the head where the state is kept and one to 
> the tail page that contains the actual chunk of data we are interested in. 
> The tail page pointer is only used for address determinations.
> 
> VM functions that manipulate the state of a page (like set_page_dirty) 
> could rely on only getting page heads.

Maybe.  Sounds ugly.  "would": so your patches remain just an RFC?

Hugh
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