linux-kernel - Re: [RFC] propagate gfp_t to page table alloc functions

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Date:	Fri, 27 Apr 2012 11:24:32 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
CC:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nick Piggin <npiggin@...il.com>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC] propagate gfp_t to page table alloc functions

On 04/26/2012 09:20 AM, David Rientjes wrote:

> On Thu, 26 Apr 2012, KAMEZAWA Hiroyuki wrote:
> 
>>> Or do we instead do this:
>>>
>>> -	some_function(foo, bar, GFP_NOIO);
>>> +	old_gfp = set_current_gfp(GFP_NOIO);
>>> +	some_function(foo, bar);
>>> +	set_current_gfp(old_gfp);
>>>
>>> So the rule is "if the code was using an explicit GFP_foo then convert
>>> it to use set_current_gfp().  If the code was receiving a gfp_t
>>> variable from the caller then delete that arg".
>>>
>>> Or something like that.  It's all too hopelessly impractical to bother
>>> discussing - 20 years too late!
>>>
>>>
>>> otoh, maybe a constrained version of this could be used to address the
>>> vmalloc() problem alone.
>>>
>>
>> Yes, I think it will be good start.
>>
> 
> Maybe a per-thread_info variant of gfp_allowed_mask?  So Andrew's 
> set_current_gfp() becomes set_current_gfp_allowed() that does
> 
> 	void set_current_gfp_allowed(gfp_t gfp_mask)
> 	{
> 		current->gfp_allowed = gfp_mask & gfp_allowed_mask;
> 	}
> 
> and then the page allocator does
> 
> 	gfp_mask &= current->gfp_allowed;
> 
> rather than how it currently does
> 
> 	gfp_mask &= gfp_allowed_mask;
> 
> and then the caller of set_current_gfp_allowed() cleans up with 
> set_current_gfp_allowed(__GFP_BITS_MASK).


Caller should restore old gfp_mask instead of __GFP_BITS_MASK in case of
nesting.And how do we care of atomic context?

I was about to add warning in __vmalloc internal if caller uses GFP_NOIO, GFP_NOFS, GFP_ATOMIC
with Nick's comment and let them make to fix it. But it seems Andrew doesn't agree.

Andrew, please tell me your opinion for fixing this problem.

-- 
Kind regards,
Minchan Kim
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