Re: [PATCH v4 03/11] mm/gup: Applies counting method to monitor gup_pgd_range

Leonardo Bras
2019-10-01T17:56:19Z

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On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:
> On 9/27/19 4:40 PM, Leonardo Bras wrote:
> > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
> > monitor against THP split/collapse with the couting method, it's necessary
>
> s/couting/counting/
>

Thanks, fixed for v5.

> > to bound it with {start,end}_lockless_pgtbl_walk.
> >
> > There are dummy functions, so it is not going to add any overhead on archs
> > that don't use this method.
> >
> > Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> > ---
> > mm/gup.c | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 98f13ab37bac..7105c829cf44 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
> > int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > struct page **pages)
> > {
> > + struct mm_struct *mm;
>
> I don't think that this local variable adds any value, so let's not use it.
> Similar point in a few other patches too.

It avoids 1 deference of current->mm, it's a little performance gain.

>
> > unsigned long len, end;
> > unsigned long flags;
> > int nr = 0;
> > @@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >
> > if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> > gup_fast_permitted(start, end)) {
> > + mm = current->mm;
> > + start_lockless_pgtbl_walk(mm);
> > local_irq_save(flags);
> > gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> > local_irq_restore(flags);
> > + end_lockless_pgtbl_walk(mm);
> > }
> >
> > return nr;
> > @@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> > unsigned int gup_flags, struct page **pages)
> > {
> > unsigned long addr, len, end;
> > + struct mm_struct *mm;
>
> Same here.
>
> > int nr = 0, ret = 0;
> >
> > if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
> > @@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >
> > if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> > gup_fast_permitted(start, end)) {
> > + mm = current->mm;
> > + start_lockless_pgtbl_walk(mm);
>
> Minor: I'd like to rename this register_lockless_pgtable_walker().
>
> > local_irq_disable();
> > gup_pgd_range(addr, end, gup_flags, pages, &nr);
> > local_irq_enable();
> > + end_lockless_pgtbl_walk(mm);
>
> ...and deregister_lockless_pgtable_walker().
>

I have no problem changing the name, but I don't register/deregister
are good terms for this.

I would rather use start/finish, begin/end, and so on. Register sounds
like something more complicated than what we are trying to achieve
here.

>
> thanks,

Thank you!

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