Re: [PATCH] nommu: add page_align to mmap

[Greg Ungerer <gerg@snapgear.com>]

Hi Bob,

On 05/05/11 20:19, Bob Liu wrote:
> On Thu, May 5, 2011 at 9:38 AM, Greg Ungerer<gerg@snapgear.com>  wrote:
>> On 05/05/11 07:13, Andrew Morton wrote:
>>>
>>> On Wed, 27 Apr 2011 15:12:14 +0800
>>> Bob Liu<lliubbo@gmail.com>  áwrote:
>>>
>>>> Currently on nommu arch mmap(),mremap() and munmap() doesn't do
>>>> page_align()
>>>> which is incorrect and not consist with mmu arch.
>>>> This patch fix it.
>>>>
>>>
>>> Can you explain this fully please? áWhat was the user-observeable
>>> behaviour before the patch, and after?
>>>
>>> And some input from nommu maintainers would be nice.
>>
>> Its not obvious to me that there is a problem here. Are there
>> any issues caused by the current behavior that this fixes?
>>
>
> Yes, there is a issue.
>
> Some drivers'  mmap() function depend on (vma->vm_end - vma->start) is
> page aligned which is true on mmu arch but not on nommu.
> eg: uvc camera driver.
>
> What's more, sometimes I got munmap() error.
> The reason is split file: mm/nommu.c
>                     do {
> 1614                         if (start>  vma->vm_start) {
> 1615                                 kleave(" = -EINVAL [miss]");
> 1616                                 return -EINVAL;
> 1617                         }
> 1618                         if (end == vma->vm_end)
> 1619                                 goto erase_whole_vma;
>
> <<=====================here
> 1620                         rb = rb_next(&vma->vm_rb);
> 1621                         vma = rb_entry(rb, struct vm_area_struct, vm_rb);
> 1622                 } while (rb);
> 1623                 kleave(" = -EINVAL [split file]");
>
> Because end is not page aligned (passed into from userspace) while
> some unknown reason
> vma->vm_end is aligned,  this loop will fail and -EINVAL[split file]
> error returned.
> But it's hard to reproduce.
>
> And in my opinion consist with mmu alway a better choice.
>
> Thanks for your review.

Ok, makes sense. Can you add some of this writeup to the patch
commit message?

Regards
Greg


------------------------------------------------------------------------
Greg Ungerer  --  Principal Engineer        EMAIL:     gerg@snapgear.com
SnapGear Group, McAfee                      PHONE:       +61 7 3435 2888
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