linux-kernel - Re: [PATCH] mm: Make {,set}page_address() static inline if WANT_PAGE

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Date:	Wed, 15 Jan 2014 15:02:14 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Guenter Roeck <linux@...ck-us.net>,
	"Michael S. Tsirkin" <mst@...hat.com>, linux-mm@...ck.org,
	linux-bcache@...r.kernel.org, Vineet Gupta <vgupta@...opsys.com>,
	sparclinux@...r.kernel.org, linux-m68k@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Make {,set}page_address() static inline if
 WANT_PAGE_VIRTUAL

On Wed, 15 Jan 2014 14:57:56 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> On Wed, 15 Jan 2014, Geert Uytterhoeven wrote:
> 
> > {,set}page_address() are macros if WANT_PAGE_VIRTUAL.
> > If !WANT_PAGE_VIRTUAL, they're plain C functions.
> > 
> > If someone calls them with a void *, this pointer is auto-converted to
> > struct page * if !WANT_PAGE_VIRTUAL, but causes a build failure on
> > architectures using WANT_PAGE_VIRTUAL (arc, m68k and sparc):
> 
> s/sparc/sparc64/
> 
> > 
> > drivers/md/bcache/bset.c: In function _____btree_sort___:
> > drivers/md/bcache/bset.c:1190: warning: dereferencing ___void *___ pointer
> > drivers/md/bcache/bset.c:1190: error: request for member ___virtual___ in something not a structure or union
> > 
> > Convert them to static inline functions to fix this. There are already
> > plenty of  users of struct page members inside <linux/mm.h>, so there's no
> > reason to keep them as macros.
> > 
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> 
> Tested-by: David Rientjes <rientjes@...gle.com>
> 
> I'm cringing at the page_address(mempool_alloc(..., GFP_NOIO)) in 
> drivers/md/bcache/bset.c, though.

Yes, I was staring suspiciously at that as well.  Ended up deciding
that I wouldn't have coded it that way, but we should support the
casting.

>  It's relying on that fact that 
> mempool_alloc() can never return NULL if __GFP_WAIT but I think this could 
> have all been avoided with
> 
> 	struct page *page = mempool_alloc(state->pool, GFP_NOIO);
> 	out = page_address(page);
> 
> instead of burying the mempool_alloc() in page_address() for what I think 
> is cleaner code.  Owell, it fixes the issue.

And that would make the later virt_to_page() unnecessary.
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