linux-kernel - Re: [PATCH V4] mm/thp: Allocate transparent hugepages on local node

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Date:	Mon, 26 Jan 2015 13:40:31 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4] mm/thp: Allocate transparent hugepages on local node

On 01/26/2015 01:13 PM, Kirill A. Shutemov wrote:
> On Mon, Jan 26, 2015 at 12:41:55PM +0100, Vlastimil Babka wrote:
>> On 01/21/2015 01:48 AM, Andrew Morton wrote:
>> > On Tue, 20 Jan 2015 17:04:31 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
>> >> + * Should be called with the mm_sem of the vma hold.
>> > 
>> > That's a pretty cruddy sentence, isn't it?  Copied from
>> > alloc_pages_vma().  "vma->vm_mm->mmap_sem" would be better.
>> > 
>> > And it should tell us whether mmap_sem required a down_read or a
>> > down_write.  What purpose is it serving?
>> 
>> This is already said for mmap_sem further above this comment line, which
>> should be just deleted (and from alloc_hugepage_vma comment too).
>> 
>> >> + *
>> >> + */
>> >> +struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
>> >> +				unsigned long addr, int order)
>> > 
>> > This pointlessly bloats the kernel if CONFIG_TRANSPARENT_HUGEPAGE=n?
>> > 
>> > 
>> > 
>> > --- a/mm/mempolicy.c~mm-thp-allocate-transparent-hugepages-on-local-node-fix
>> > +++ a/mm/mempolicy.c
>> 
>> How about this cleanup on top? I'm not fully decided on the GFP_TRANSHUGE test.
>> This is potentially false positive, although I doubt anything else uses the same
>> gfp mask bits.
> 
> This info on gfp mask should be in commit message.

Right. Wanted to get some consensus first.

> And what about WARN_ON_ONCE() if we the matching bits with
> !TRANSPARENT_HUGEPAGE?

Hmm, can't say I like that, but could work.

>> 
>> Should "hugepage" be extra bool parameter instead? Should I #ifdef the parameter
>> only for CONFIG_TRANSPARENT_HUGEPAGE, or is it not worth the ugliness?
> 
> Do we have spare gfp bit? ;)

Seems we have defined 24 out of 32. Not too much to spare, and the use case here
is very narrow.
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