Subject: Re: [RESEND PATCH v3 1/2] mm: Introducing arch_remap hook

From: Laurent Dufour <[email-masked]>
Date: Mon, 13 Apr 2015 15:21:14 +0200

On 13/04/2015 15:13, Kirill A. Shutemov wrote:
> On Mon, Apr 13, 2015 at 02:41:22PM +0200, Laurent Dufour wrote:
>> On 13/04/2015 13:58, Kirill A. Shutemov wrote:
>>> On Mon, Apr 13, 2015 at 11:56:27AM +0200, Laurent Dufour wrote:
>>>> Some architecture would like to be triggered when a memory area is moved
>>>> through the mremap system call.
>>>>
>>>> This patch is introducing a new arch_remap mm hook which is placed in the
>>>> path of mremap, and is called before the old area is unmapped (and the
>>>> arch_unmap hook is called).
>>>>
>>>> The architectures which need to call this hook should define
>>>> __HAVE_ARCH_REMAP in their asm/mmu_context.h and provide the arch_remap
>>>> service with the following prototype:
>>>> void arch_remap(struct mm_struct *mm,
>>>>                 unsigned long old_start, unsigned long old_end,
>>>>                 unsigned long new_start, unsigned long new_end);
>>>>
>>>> Signed-off-by: Laurent Dufour <[email-masked]>
>>>> Reviewed-by: Ingo Molnar <[email-masked]>
>>>> ---
>>>>  mm/mremap.c | 19 +++++++++++++------
>>>>  1 file changed, 13 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/mm/mremap.c b/mm/mremap.c
>>>> index 2dc44b1cb1df..009db5565893 100644
>>>> --- a/mm/mremap.c
>>>> +++ b/mm/mremap.c
>>>> @@ -25,6 +25,7 @@
>>>>  
>>>>  #include <asm/cacheflush.h>
>>>>  #include <asm/tlbflush.h>
>>>> +#include <asm/mmu_context.h>
>>>>  
>>>>  #include "internal.h"
>>>>  
>>>> @@ -286,13 +287,19 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>>>>  		old_len = new_len;
>>>>  		old_addr = new_addr;
>>>>  		new_addr = -ENOMEM;
>>>> -	} else if (vma->vm_file && vma->vm_file->f_op->mremap) {
>>>> -		err = vma->vm_file->f_op->mremap(vma->vm_file, new_vma);
>>>> -		if (err < 0) {
>>>> -			move_page_tables(new_vma, new_addr, vma, old_addr,
>>>> -					 moved_len, true);
>>>> -			return err;
>>>> +	} else {
>>>> +		if (vma->vm_file && vma->vm_file->f_op->mremap) {
>>>> +			err = vma->vm_file->f_op->mremap(vma->vm_file, new_vma);
>>>> +			if (err < 0) {
>>>> +				move_page_tables(new_vma, new_addr, vma,
>>>> +						  old_addr, moved_len, true);
>>>> +				return err;
>>>> +			}
>>>>  		}
>>>> +#ifdef __HAVE_ARCH_REMAP
>>>
>>> It would be cleaner to provide dummy arch_remap() for !__HAVE_ARCH_REMAP
>>> in some generic header.
>>
>> The idea was to not impact all the architectures as arch_unmap(),
>> arch_dup_mmap() or arch_exit_mmap() implies.
>>
>> I look at the headers where such a dummy arch_remap could be put but I
>> can't figure out one which will not impact all the architecture.
>> What about defining a dummy service earlier in mm/remap.c in the case
>> __HAVE_ARCH_REMAP is not defined ?
>> Something like :
>> #ifndef __HAVE_ARCH_REMAP
>> static inline void void arch_remap(struct mm_struct *mm,
>>                                    unsigned long old_start,
>>                                    unsigned long old_end,
>>                                    unsigned long new_start,
>>                                    unsigned long new_end)
>> {
>> }
>> #endif
> 
> Or just #define arch_remap(...) do { } while (0)
> 

I guessed you wanted the arch_remap() prototype to be exposed somewhere
in the code.

To be honest, I can't find the benefit of defining a dummy arch_remap()
in mm/remap.c if __HAVE_ARCH_REMAP is not defined instead of calling it
in move_vma if __HAVE_ARCH_REMAP is defined.
Is it really what you want ?

Thanks,
Laurent.