linux-kernel - Re: [RFC] x86: gup

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Date:	Wed, 24 Jun 2009 15:46:10 +0200
From:	Brice Goglin <Brice.Goglin@...ia.fr>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] x86: gup_fast() batch limit

Any news about this patch?

Brice



Nick Piggin wrote:
> On Saturday 28 March 2009 23:46:14 Peter Zijlstra wrote:
>   
>> On Sat, 2009-03-28 at 13:22 +0100, Peter Zijlstra wrote:
>>     
>>> I'm not really trusting my brain today, but something like the below
>>> should work I think.
>>>
>>> Nick, any thoughts?
>>>
>>> Not-Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>>> ---
>>>  arch/x86/mm/gup.c |   24 +++++++++++++++++++++---
>>>  1 files changed, 21 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
>>> index be54176..4ded5c3 100644
>>> --- a/arch/x86/mm/gup.c
>>> +++ b/arch/x86/mm/gup.c
>>> @@ -11,6 +11,8 @@
>>>
>>>  #include <asm/pgtable.h>
>>>
>>> +#define GUP_BATCH	32
>>> +
>>>  static inline pte_t gup_get_pte(pte_t *ptep)
>>>  {
>>>  #ifndef CONFIG_X86_PAE
>>> @@ -91,7 +93,8 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned
>>> long addr, get_page(page);
>>>  		pages[*nr] = page;
>>>  		(*nr)++;
>>> -
>>> +		if (*nr > GUP_BATCH)
>>> +			break;
>>>  	} while (ptep++, addr += PAGE_SIZE, addr != end);
>>>  	pte_unmap(ptep - 1);
>>>
>>> @@ -157,6 +160,8 @@ static int gup_pmd_range(pud_t pud, unsigned long
>>> addr, unsigned long end, if (!gup_pte_range(pmd, addr, next, write,
>>> pages, nr))
>>>  				return 0;
>>>  		}
>>> +		if (*nr > GUP_BATCH)
>>> +			break;
>>>  	} while (pmdp++, addr = next, addr != end);
>>>
>>>  	return 1;
>>> @@ -214,6 +219,8 @@ static int gup_pud_range(pgd_t pgd, unsigned long
>>> addr, unsigned long end, if (!gup_pmd_range(pud, addr, next, write,
>>> pages, nr))
>>>  				return 0;
>>>  		}
>>> +		if (*nr > GUP_BATCH)
>>> +			break;
>>>  	} while (pudp++, addr = next, addr != end);
>>>
>>>  	return 1;
>>> @@ -226,7 +233,7 @@ int get_user_pages_fast(unsigned long start, int
>>> nr_pages, int write, unsigned long addr, len, end;
>>>  	unsigned long next;
>>>  	pgd_t *pgdp;
>>> -	int nr = 0;
>>> +	int batch = 0, nr = 0;
>>>
>>>  	start &= PAGE_MASK;
>>>  	addr = start;
>>> @@ -254,6 +261,7 @@ int get_user_pages_fast(unsigned long start, int
>>> nr_pages, int write, * (which we do on x86, with the above PAE
>>> exception), we can follow the * address down to the the page and take a
>>> ref on it.
>>>  	 */
>>> +again:
>>>  	local_irq_disable();
>>>  	pgdp = pgd_offset(mm, addr);
>>>  	do {
>>> @@ -262,11 +270,21 @@ int get_user_pages_fast(unsigned long start, int
>>> nr_pages, int write, next = pgd_addr_end(addr, end);
>>>  		if (pgd_none(pgd))
>>>  			goto slow;
>>> -		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
>>> +		if (!gup_pud_range(pgd, addr, next, write, pages, &batch))
>>>  			goto slow;
>>> +		if (batch > GUP_BATCH) {
>>> +			local_irq_enable();
>>> +			addr += batch << PAGE_SHIFT;
>>> +			nr += batch;
>>> +			batch = 0;
>>> +			if (addr != end)
>>> +				goto again;
>>> +		}
>>>  	} while (pgdp++, addr = next, addr != end);
>>>  	local_irq_enable();
>>>
>>> +	nr += batch;
>>> +
>>>  	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
>>>  	return nr;
>>>       
>> Would also need the following bit:
>>
>> @@ -274,6 +292,7 @@ int get_user_pages_fast(unsigned long start, int
>> nr_pages, int write, int ret;
>>
>>  slow:
>> +		nr += batch;
>>  		local_irq_enable();
>>  slow_irqon:
>>  		/* Try to get the remaining pages with get_user_pages */
>>     
>
>
> Yeah something like this would be fine (and welcome). And we can
> remove the XXX comment in there too. I would suggest 64 being a
> reasonable value simply because that's what direct IO does.
>
> Implementation-wise, why not just break "len" into chunks in the
> top level function rather than add branches all down the call
> chain?
>   

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