Re: [PATCH v2 6/9] mm: free user PTE page table pages

[David Hildenbrand]

On 01.09.21 18:16, Jason Gunthorpe wrote:
> On Wed, Sep 01, 2021 at 06:13:07PM +0200, David Hildenbrand wrote:
> > On 01.09.21 17:32, Jason Gunthorpe wrote:
> > > On Wed, Sep 01, 2021 at 03:57:09PM +0200, David Hildenbrand wrote:
> > > > On 01.09.21 15:53, Jason Gunthorpe wrote:
> > > > > On Thu, Aug 19, 2021 at 11:18:55AM +0800, Qi Zheng wrote:
> > > > >
> > > > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > > > index 2630ed1bb4f4..30757f3b176c 100644
> > > > > > +++ b/mm/gup.c
> > > > > > @@ -500,6 +500,9 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> > > > > >     	if (unlikely(pmd_bad(*pmd)))
> > > > > >     		return no_page_table(vma, flags);
> > > > > > +	if (!pte_try_get(mm, pmd))
> > > > > > +		return no_page_table(vma, flags);
> > > > > > +
> > > > > >     	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
> > > > >
> > > > > This is not good on a performance path, the pte_try_get() is
> > > > > locking/locking the same lock that pte_offset_map_lock() is getting.
> > > >
> > > > Yes, and we really need patch #8, anything else is just confusing reviewers.
> > >
> > > It is a bit better with patch 8, but it is still not optimal, we don't
> > > need to do the atomic work at all if the entire ptep is accessed while
> > > locked. So the above is stil not what I would expect here, even with
> > > RCU.
> > >
> > > eg I would expect that this kind of change would work first with the
> > > existing paired acessors, ie
> > >
> > > 	pte = pte_offset_map(pmd, address);
> > > 	pte_unmap(pte);
> > >
> > > Should handle the refcount under the covers, and same kind of idea for
> > > the _locked/_unlocked varient.
> >
> > See my other mail.
>
> Do you have a reference?

Reply to the other mail you just send.

> > > Only places that don't already use that pairing should get modified.
> > >
> > > To do this we have to extend the API so that pte_offset_map() can
> > > fail, or very cleverly return some kind of global non-present pte page
> > > (I wonder if the zero page would work?)
> >
> > I explored both ideas (returning NULL, return a specially prepared page) and
> > it didn't work in some cases where we unmap+remap etc.
>
> I wouldn't think it works everywhere, bit it works in a lot of places,
> and it is a heck of a lot better than what is proposed here. I'd
> rather see the places that can use it be moved, and the few places
> that can't be opencoded.

Well, I used ptep_get_map_lock() and friends. But hacking directly into 
ptep_map_lock() and friends wasn't possible due to all the corner cases.

--
Thanks,

David / dhildenb