linux-kernel - Re: [PATCHv2, RFC 07/30] thp, mm: introduce mapping_can_have

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Date:	Fri, 22 Mar 2013 07:44:56 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2, RFC 07/30] thp, mm: introduce mapping_can_have_hugepages()
 predicate

On 03/22/2013 03:12 AM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
>>> +static inline bool mapping_can_have_hugepages(struct address_space *m)
>>> +{
>>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>>> +		gfp_t gfp_mask = mapping_gfp_mask(m);
>>> +		return !!(gfp_mask & __GFP_COMP);
>>> +	}
>>> +
>>> +	return false;
>>> +}
>>
>> I did a quick search in all your patches and don't see __GFP_COMP
>> getting _set_ anywhere.  Am I missing something?
> 
> __GFP_COMP is part of GFP_TRANSHUGE. We set it for ramfs in patch 20/30.

That's a bit non-obvious.  For a casual observer, it _seems_ like you
should just be setting and checking GFP_TRANSHUGE directly.  It looks
like you were having some problems with __GFP_MOVABLE and masked it out
of GFP_TRANSHUGE and that has cascaded over to _this_ check.

I _think_ the right thing to do is add a comment up there in
mapping_can_have_hugepages() that does (GFP_TRANSHUGE & ~__GFP_MOVABLE),
and adds a TODO in the code and patch comments to clean it up once
ramfs_get_inode() gets fixed up too.
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