Re: [PATCH] mmap : save some cycles for the shared anonymous mapping

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Fri, 11 Sep 2009, Andrew Morton wrote:
> On Fri, 11 Sep 2009 09:52:46 +0800
> Huang Shijie <shijie8@gmail.com> wrote:
> 
> > The shmem_zere_setup() does not change vm_start, pgoff or vm_flags,
> > only some drivers change them (such as /driver/video/bfin-t350mcqb-fb.c).
> > 
> > Moving these codes to a more proper place to save cycles for shared
> > anonymous mapping.

(Actually it's saving them for any !file mapping.
 Though I doubt it's a significant saving myself.)

> > 
> > Signed-off-by: Huang Shijie <shijie8@gmail.com>

Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

> > ---
> >  mm/mmap.c |   18 +++++++++---------
> >  1 files changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 8101de4..840e91e 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1195,21 +1195,21 @@ munmap_back:
> >  			goto unmap_and_free_vma;
> >  		if (vm_flags & VM_EXECUTABLE)
> >  			added_exe_file_vma(mm);
> > +
> > +		/* Can addr have changed??
> > +		 *
> > +		 * Answer: Yes, several device drivers can do it in their
> > +		 *         f_op->mmap method. -DaveM
> > +		 */
> > +		addr = vma->vm_start;
> > +		pgoff = vma->vm_pgoff;
> > +		vm_flags = vma->vm_flags;
> >  	} else if (vm_flags & VM_SHARED) {
> >  		error = shmem_zero_setup(vma);
> >  		if (error)
> >  			goto free_vma;
> >  	}
> >  
> > -	/* Can addr have changed??
> > -	 *
> > -	 * Answer: Yes, several device drivers can do it in their
> > -	 *         f_op->mmap method. -DaveM
> > -	 */
> > -	addr = vma->vm_start;
> > -	pgoff = vma->vm_pgoff;
> > -	vm_flags = vma->vm_flags;
> > -
> >  	if (vma_wants_writenotify(vma))
> >  		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
> >  
> 
> hm, maybe we should nuke those locals and just use vma->foo everywhere.
> 
> Local variable pgoff never gets used again anyway.

I think it was me who Nak'ed an earlier patch to remove the update
of pgoff, out of fear that we might add a later reference sometime
in future, and not notice for a long time that it then needed that
update again.

addr and pgoff start off as args to do_mmap_pgoff(), so we'd better
not nuke them!  And if we changed all the lines below that point to
refer to vma->vm_start and vma->vm_flags, I think there's still a
danger we'd unthinkingly add a reference to addr or vm_flags later.

If any change is to be made here, I think I prefer Shijie's:
shmem_zero_setup isn't likely to change to modify any of those,
and that patch has the great virtue of retaining DaveM's comment,
which draws attention to the issue.

Hugh

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