linux-kernel - Re: Pagecache: find_or_create_page does not call a proper page allocator function

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Message-Id: <20070424135348.4e69ac23.akpm@linux-foundation.org>
Date:	Tue, 24 Apr 2007 13:53:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org, pj@....com
Subject: Re: Pagecache: find_or_create_page does not call a proper page
 allocator function

On Tue, 24 Apr 2007 13:44:50 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Tue, 24 Apr 2007, Andrew Morton wrote:
> 
> > > I would say that the filesystem is broke if it has such expectations 
> > > regardless of page migration.
> > 
> > Others disagree ;)
> > 
> > The filesystem has *told* the core kernel what its allocation constraints
> > are by setting up mapping_gfp_mask().  If the core kernel stops honouring
> > that request then it is core kernel which is broken.
> 
> Then I think we should disable page migration for allocations that do not 
> allow access to the policy zone. That would fix it.

Can't we use mapping_gfp_mask() when allocating the destination page?

It would be better to do so, really.  Who knows, mapping_gfp_mask() might
be extented in the future to say "I want GFP_NOIO" or something.  Or a
filesystem might specify GFP_KERNEL for regular pagecache pages or
whatever.

Generally, the interface is "address_space tells core kernel how to
allocate its pages", and to be nice we should honour that in all places
where we allocate a page for an address_space.

If we'd had any brains we would have implemented this function as an
address_space_operations callback, but we don't so we didn't.

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