linux-kernel - Re: [PATCHv4 27/39] x86-64, mm: proper alignment mappings with hugepages

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Message-ID: <51C9C960.6070706@sr71.net>
Date:	Tue, 25 Jun 2013 09:46:24 -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: [PATCHv4 27/39] x86-64, mm: proper alignment mappings with hugepages

On 06/25/2013 07:56 AM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
>>> +static inline unsigned long mapping_align_mask(struct address_space *mapping)
>>> +{
>>> +	if (mapping_can_have_hugepages(mapping))
>>> +		return PAGE_MASK & ~HPAGE_MASK;
>>> +	return get_align_mask();
>>> +}
>>
>> get_align_mask() appears to be a bit more complicated to me than just a
>> plain old mask.  Are you sure you don't need to pick up any of its
>> behavior for the mapping_can_have_hugepages() case?
> 
> get_align_mask() never returns more strict mask then we do in
> mapping_can_have_hugepages() case.
> 
> I can modify it this way:
> 
>         unsigned long mask = get_align_mask();
> 
>         if (mapping_can_have_hugepages(mapping))
>                 mask &= PAGE_MASK & ~HPAGE_MASK;
>         return mask;
> 
> But it looks more confusing for me. What do you think?

Personally, I find that a *LOT* more clear.  The &= pretty much spells
out what you said in your explanation: get_align_mask()'s mask can only
be made more strict when we encounter a huge page.

The relationship between the two masks is not apparent at all in your
original code.  This is all nitpicking though, I just wanted to make
sure you'd considered if you were accidentally changing behavior.
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