linux-kernel - Re: [PATCH 1/4] zsmalloc: remove x86 dependency

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Message-ID: <4FFD2AE6.70609@kernel.org>
Date:	Wed, 11 Jul 2012 16:27:34 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
CC:	devel@...verdev.osuosl.org,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/4] zsmalloc: remove x86 dependency

On 07/11/2012 12:29 AM, Seth Jennings wrote:
> On 07/09/2012 09:21 PM, Minchan Kim wrote:
>> On 07/03/2012 06:15 AM, Seth Jennings wrote:
> <snip>
>>> +static void zs_copy_map_object(char *buf, struct page *firstpage,
>>> +				int off, int size)
>>
>> firstpage is rather misleading.
>> As you know, we use firstpage term for real firstpage of zspage but
>> in case of zs_copy_map_object, it could be a middle page of zspage.
>> So I would like to use "page" instead of firstpage.
> 
> Accepted.
> 
>>> +{
>>> +	struct page *pages[2];
>>> +	int sizes[2];
>>> +	void *addr;
>>> +
>>> +	pages[0] = firstpage;
>>> +	pages[1] = get_next_page(firstpage);
>>> +	BUG_ON(!pages[1]);
>>> +
>>> +	sizes[0] = PAGE_SIZE - off;
>>> +	sizes[1] = size - sizes[0];
>>> +
>>> +	/* disable page faults to match kmap_atomic() return conditions */
>>> +	pagefault_disable();
>>
>> If I understand your intention correctly, you want to prevent calling
>> this function on non-atomic context. Right?
> 
> This is moved to zs_map_object() in a later patch, but the
> point is to provide uniform return conditions, regardless of
> whether the object to be mapped is contained in a single
> page or spans two pages.  kmap_atomic() disables page
> faults, so I did it here to create symmetry.  The result is

The one I want to comment out is why we should disable page fault.
ie, if we don't disable page fault, what's happen?

As I read the comment of kmap_atomic about pagefault_disable, 
it seems that for preventing reentrant bug in preemptive kernel
while it catch page fault during atomic context in non-preemptive kernel.
But I'm not sure so Ccing Peter.


> that zs_map_object always returns with preemption and page
> faults disabled.
> 
> Also, Greg already merged these patches so I'll have to
> incorporate these changes as a separate patch.
> 
> Thanks,
> Seth
> 


-- 
Kind regards,
Minchan Kim


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