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

From: Jason Gunthorpe <jgg@nvidia.com>
To: David Hildenbrand <david@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>,
	akpm@linux-foundation.org, tglx@linutronix.de,
	hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
	kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, songmuchun@bytedance.com
Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages
Date: Wed, 1 Sep 2021 13:16:13 -0300	[thread overview]
Message-ID: <20210901161613.GN1721383@nvidia.com> (raw)
In-Reply-To: <7789261d-6a64-c47b-be6c-c9be680e5d33@redhat.com>

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?

> > 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.

Jason