linux-kernel - Re: [patch] mm, page_alloc: make __GFP

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Date:	Mon, 9 Dec 2013 15:22:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, page_alloc: make __GFP_NOFAIL really not fail

On Mon, 9 Dec 2013 13:56:37 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> __GFP_NOFAIL specifies that the page allocator cannot fail to return
> memory.  Allocators that call it may not even check for NULL upon
> returning.
> 
> It turns out GFP_NOWAIT | __GFP_NOFAIL or GFP_ATOMIC | __GFP_NOFAIL can
> actually return NULL.  More interestingly, processes that are doing
> direct reclaim and have PF_MEMALLOC set may also return NULL for any
> __GFP_NOFAIL allocation.

__GFP_NOFAIL is a nasty thing and making it pretend to work even better
is heading in the wrong direction, surely?  It would be saner to just
disallow these even-sillier combinations.  Can we fix up the current
callers then stick a WARN_ON() in there?

> This patch fixes it so that the page allocator never actually returns
> NULL as expected for __GFP_NOFAIL.  It turns out that no code actually
> does anything as crazy as GFP_ATOMIC | __GFP_NOFAIL currently, so this
> is more for correctness than a bug fix for that issue.
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