Re: [PATCH] mm: avoid unnecessary page table walk for __get_user_pages

[John Hubbard]

On 9/7/22 16:34, Andrew Morton wrote:
> On Tue, 6 Sep 2022 10:44:01 +0800 Chen Wandun <chenwandun@xxxxxxxxxx> wrote:
>
> > It is no need to walk page table and find pages if faultin_page success
> > and __get_user_pages does't care pages at all, so directly handle next
> > page.
> >
> > ...
> >
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1198,7 +1198,10 @@ static long __get_user_pages(struct mm_struct *mm,
> >   					   PTR_ERR(page) == -EMLINK, locked);
> >   			switch (ret) {
> >   			case 0:
> > -				goto retry;
> > +				if (pages)
> > +					goto retry;
> > +				else
> > +					goto next_page;

Wow, good point. And it's been doing that for a *long* time!

> >   			case -EBUSY:
> >   			case -EAGAIN:
> >   				ret = 0;
>
> Looks right to me.  I redid the changelog somewhat:
>
> : There is no need to walk page tables and find pages if faultin_page)_
> : succeeds and __get_user_pages)_ doesn't care about populating the pages[]
> : array.  So directly go on to handle the next page.

With that re-worded commit description, please feel to add

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>


thanks,
--
John Hubbard
NVIDIA