netdev - Re: [RFC 2/4] jbd2: revert must-not-fail allocation loops back to GFP

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Date:	Mon, 2 Mar 2015 12:33:21 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>, Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	"David S. Miller" <davem@...emloft.net>,
	sparclinux@...r.kernel.org, Vipul Pandya <vipul@...lsio.com>,
	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/4] jbd2: revert must-not-fail allocation loops back to
 GFP_NOFAIL

On Mon, 2 Mar 2015, Michal Hocko wrote:

> This basically reverts 47def82672b3 (jbd2: Remove __GFP_NOFAIL from jbd2
> layer). The deprecation of __GFP_NOFAIL was a bad choice because it led
> to open coding the endless loop around the allocator rather than
> removing the dependency on the non failing allocation. So the
> deprecation was a clear failure and the reality tells us that
> __GFP_NOFAIL is not even close to go away.
> 
> It is still true that __GFP_NOFAIL allocations are generally discouraged
> and new uses should be evaluated and an alternative (pre-allocations or
> reservations) should be considered but it doesn't make any sense to lie
> the allocator about the requirements. Allocator can take steps to help
> making a progress if it knows the requirements.
> 

The changelog should state that this only changes the source code, there 
is no functional change since alloc_buffer_head() and 
kmem_cache_zalloc(transaction_cache) are already implicitly nofail due to 
the allocation order.  The failure code added by the commit you cite are 
never executed.

I agree that if the implementation of the page allocator were to change 
with respect to PAGE_ALLOC_COSTLY_ORDER that we'd need __GFP_NOFAIL and 
that such an allocation is better handled in the page allocator.

> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Acked-by: David Rientjes <rientjes@...gle.com>

GFP_NOFS|__GFP_NOFAIL is scary.
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