Subject: Re: [PATCH v2 07/18] infiniband: set FOLL_PIN, FOLL_LONGTERM via pin_longterm_pages*()

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 4 Nov 2019 22:32:35 -0400

On Mon, Nov 04, 2019 at 02:03:43PM -0800, John Hubbard wrote:
> On 11/4/19 12:57 PM, Jason Gunthorpe wrote:
> > On Mon, Nov 04, 2019 at 12:48:13PM -0800, John Hubbard wrote:
> >> On 11/4/19 12:33 PM, Jason Gunthorpe wrote:
> >> ...
> >>>> diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> >>>> index 24244a2f68cc..c5a78d3e674b 100644
> >>>> +++ b/drivers/infiniband/core/umem.c
> >>>> @@ -272,11 +272,10 @@ struct ib_umem *ib_umem_get(struct ib_udata *udata, unsigned long addr,
> >>>>  
> >>>>  	while (npages) {
> >>>>  		down_read(&mm->mmap_sem);
> >>>> -		ret = get_user_pages(cur_base,
> >>>> +		ret = pin_longterm_pages(cur_base,
> >>>>  				     min_t(unsigned long, npages,
> >>>>  					   PAGE_SIZE / sizeof (struct page *)),
> >>>> -				     gup_flags | FOLL_LONGTERM,
> >>>> -				     page_list, NULL);
> >>>> +				     gup_flags, page_list, NULL);
> >>>
> >>> FWIW, this one should be converted to fast as well, I think we finally
> >>> got rid of all the blockers for that?
> >>>
> >>
> >> I'm not aware of any blockers on the gup.c end, anyway. The only broken thing we
> >> have there is "gup remote + FOLL_LONGTERM". But we can do "gup fast + LONGTERM". 
> > 
> > I mean the use of the mmap_sem here is finally in a way where we can
> > just delete the mmap_sem and use _fast
> >  
> > ie, AFAIK there is no need for the mmap_sem to be held during
> > ib_umem_add_sg_table()
> > 
> > This should probably be a standalone patch however
> > 
> 
> Yes. Oh, actually I guess the patch flow should be: change to 
> get_user_pages_fast() and remove the mmap_sem calls, as one patch. And then change 
> to pin_longterm_pages_fast() as the next patch. Otherwise, the internal fallback
> from _fast to slow gup would attempt to take the mmap_sem (again) in the same
> thread, which is not good. :)
> 
> Or just defer the change until after this series. Either way is fine, let me
> know if you prefer one over the other.
> 
> The patch itself is trivial, but runtime testing to gain confidence that
> it's solid is much harder. Is there a stress test you would recommend for that?
> (I'm not promising I can quickly run it yet--my local IB setup is still nascent 
> at best.)

If you make a patch we can probably get it tested, it is something
we should do I keep forgetting about.

Jason