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

From: "Kirill A. Shutemov" <[email-masked]>
Date: Mon, 13 Apr 2015 17:02:19 +0300

On Mon, Apr 13, 2015 at 04:35:21PM +0300, Pavel Emelyanov wrote:
> On 04/13/2015 04:21 PM, Laurent Dufour wrote:
> > 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 ?
> 
> I think Kirill meant something like e.g. the arch_enter_lazy_mmu_mode()
> is implemented and called in mm/mremap.c -- the "generic" part is in the
> include/asm-generic/pgtable.h and those architectures willing to have
> their own implementation are in arch/$arch/...
> 
> Kirill, if I'm right with it, can you suggest the header where to put
> the "generic" mremap hook's (empty) body?

I initially thought it would be enough to put it into
<asm-generic/mmu_context.h>, expecting it works as
<asm-generic/pgtable.h>. But that's not the case.

It probably worth at some point rework all <asm/mmu_context.h> to include
<asm-generic/mmu_context.h> at the end as we do for <asm/pgtable.h>.
But that's outside the scope of the patchset, I guess.

I don't see any better candidate for such dummy header. :-/

-- 
 Kirill A. Shutemov