linux-kernel - Re: [RFC 0/2] mapping_gfp_mask from the page fault path

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Message-ID: <20150603134204.GC16201@dhcp22.suse.cz>
Date:	Wed, 3 Jun 2015 15:42:04 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org, david@...morbit.com,
	neilb@...e.de, hannes@...xchg.org, viro@...iv.linux.org.uk,
	mgorman@...e.de, riel@...hat.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 0/2] mapping_gfp_mask from the page fault path

On Wed 03-06-15 22:04:22, Tetsuo Handa wrote:
[...]
> Michal Hocko wrote:
> > Initialize the default to (mapping_gfp_mask | GFP_IOFS) because this
> > should be safe from the page fault path normally. Why do we care
> > about mapping_gfp_mask at all then? Because this doesn't hold only
> > reclaim protection flags but it also might contain zone and movability
> > restrictions (GFP_DMA32, __GFP_MOVABLE and others) so we have to respect
> > those.
> 
> [2/2] says that mapping_gfp_mask(mapping) might contain bits which are not
> in !GFP_KERNEL. If we do
> 
>   GFP_KERNEL & mapping_gfp_mask(mapping)
> 
> we will drop such bits and will cause problems.

No we won't.

> Thus, "GFP_KERNEL"
> in patch [1/1] should be replaced with "mapping_gfp_mask(mapping)" than
> "GFP_KERNEL & mapping_gfp_mask(mapping)" ?

Those gfp_masks are for LRU handling and that is GFP_KERNEL by
default. We only need to drop those which are not compatible with
mapping_gfp_mask. We do not care about __GFP_MOVABLE, GFP_DMA32 etc...
-- 
Michal Hocko
SUSE Labs
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