Re: [PATCH] x86: do not overrun page table ranges in gup

[Johannes Weiner]

Hi,

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue, 29 Jul 2008, Johannes Weiner wrote:
>> 
>> Actually, I think the prettier fix would be to just establish that
>> garuantee:
>>
>> --- a/arch/x86/mm/gup.c
>> +++ b/arch/x86/mm/gup.c
>> @@ -223,7 +223,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>  			struct page **pages)
>>  {
>>  	struct mm_struct *mm = current->mm;
>> -	unsigned long end = start + (nr_pages << PAGE_SHIFT);
>> +	unsigned long end = PAGE_ALIGN(start + (nr_pages << PAGE_SHIFT));
>
> Umm. 'end' is guaranteed to be page-aligned if 'start' is. 
>
> So if this makes a difference, that implies that _start_ isn't 
> page-aligned, and then you when you add PAGE_SIZE to 'addr', you are going 
> to miss 'end' again.
>
> So no, the right fix would be to align 'start' first, which means that 
> everything else (including 'end') will be page-aligned. Aligning just one 
> or the other is very very wrong.

Blah, I just should call it a day.  Thanks for the explanation, you are
right.

	Hannes
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