linux-kernel - Re: [PATCH v2] tmpfs not interleaving properly

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Message-ID: <20120619232102.GA5698@gulag1.americas.sgi.com>
Date:	Tue, 19 Jun 2012 18:21:02 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Nathan Zimmer <nzimmer@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, hughd@...gle.com,
	npiggin@...il.com, cl@...ux.com, lee.schermerhorn@...com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	stable@...r.kernel.org, riel@...hat.com
Subject: Re: [PATCH v2] tmpfs not interleaving properly

On Fri, Jun 01, 2012 at 01:22:15PM -0400, KOSAKI Motohiro wrote:
> (6/1/12 10:24 AM), Nathan Zimmer wrote:
>> On Thu, May 31, 2012 at 04:35:53PM -0400, KOSAKI Motohiro wrote:
>>> (5/31/12 4:25 PM), Andrew Morton wrote:
>>>> On Thu, 31 May 2012 16:09:15 -0400
>>>> KOSAKI Motohiro<kosaki.motohiro@...il.com>   wrote:
>>>>
>>>>>> --- a/mm/shmem.c
>>>>>> +++ b/mm/shmem.c
>>>>>> @@ -929,7 +929,7 @@ static struct page *shmem_alloc_page(gfp_t gfp,
>>>>>>     	/*
>>>>>>     	 * alloc_page_vma() will drop the shared policy reference
>>>>>>     	 */
>>>>>> -	return alloc_page_vma(gfp,&pvma, 0);
>>>>>> +	return alloc_page_vma(gfp,&pvma, info->node_offset<<    PAGE_SHIFT );
>>>>>
>>>>> 3rd argument of alloc_page_vma() is an address. This is type error.
>>>>
>>>> Well, it's an unsigned long...
>>>>
>>>> But yes, it is conceptually wrong and *looks* weird.  I think we can
>>>> address that by overcoming our peculair aversion to documenting our
>>>> code, sigh.  This?
>>>
>>> Sorry, no.
>>>
>>> addr agrument of alloc_pages_vma() have two meanings.
>>>
>>> 1) interleave node seed
>>> 2) look-up key of shmem policy
>>>
>>> I think this patch break (2). shmem_get_policy(pol, addr) assume caller honor to
>>> pass correct address.
>>
>> But the pseudo vma we generated in shmem_alloc_page the vm_ops are set to NULL.
>> So get_vma_policy will return the policy provided by the pseudo vma and not reach
>> the shmem_get_policy.
>
> yes, and it is bug source. we may need to change soon. I guess the right way is
> to make vm_ops->interleave and interleave_nid uses it if povided.
>

If we provide vm_ops then won't shmem_get_policy get called?
That would be an issue since shmem_get_policy assumes vm_file is non NULL.

> btw, I don't think node_random() is good idea. it is random(pid + jiffies + cycle).
> current->cpuset_mem_spread_rotor is per-thread value. but you now need per-inode
> interleave offset. maybe, just inode addition is enough. Why do you need randomness?
>

I don't really need the randomness, the rotor should be good enough.
The correct way to get that is cpuset_mem_spread_node(), yes?

Also apologies for such a delay in my response.
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