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

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Message-ID: <Pine.LNX.4.64.0704241941421.28487@blonde.wat.veritas.com>
Date:	Tue, 24 Apr 2007 19:53:54 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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, Christoph Lameter wrote:
> On Tue, 24 Apr 2007, Hugh Dickins wrote:
> 
> > I've not yet looked at the patch under discussion, but this remark
> > prompts me...  a couple of days ago I got very worried by the various
> > hard-wired GFP_HIGHUSER allocations in mm/migrate.c and mm/mempolicy.c,
> > and wondered how those would work out if someone has a blockdev mmap'ed.
> 
> I hope you are not confused by the fact that memory policies are only
> ever applied to one zone on a node. This is either HIGHMEM or NORMAL. 
> There is no memory policy support for other than the highest zone.

I was certainly ignorant of that; but I'm not convinced it eliminates
the potential issue.  For a start, sys_move_pages seems not to involve
mempolicies at all - I don't see what prevents it migrating blockdev
pages away from the only node which has NORMAL memory.

> Metadata is not movable nor subject to memory policies.
> It will never be mapped into a process space.

Not as metadata, no.  But someone (let's hope only root, though I may
be wrong on that) can map any part of the block device into userspace.

> > yup.  wherever we dereference buffer_head.b_data we're touching
> > page_address(buffer_head.b_page) without kmapping.
> 
> Yes but before we get there we will bounce pagecache pages into an area 
> where we do not need kmap.

Again, I'm not convinced: bouncing gets done for the I/O,
but where is it done to meet the filesystem's expectations?

On the other hand, as I said, I've seen no problem myself in practice.
However, if there is no problem, why do block devices demand GFP_USER?

Hugh
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