linux-kernel - Re: [PATCH 3/3] sgi-gru: Use __get_user_pages_fast in atomic_pte

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Date:   Mon, 22 Jul 2019 23:23:11 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     arnd@...db.de, sivanich@....com, gregkh@...uxfoundation.org,
        ira.weiny@...el.com, jglisse@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/3] sgi-gru: Use __get_user_pages_fast in
 atomic_pte_lookup

On Sun, Jul 21, 2019 at 07:32:36PM -0700, John Hubbard wrote:
> On 7/21/19 8:58 AM, Bharath Vedartham wrote:
> > *pte_lookup functions get the physical address for a given virtual
> > address by getting a physical page using gup and use page_to_phys to get
> > the physical address.
> > 
> > Currently, atomic_pte_lookup manually walks the page tables. If this
> > function fails to get a physical page, it will fall back too
> > non_atomic_pte_lookup to get a physical page which uses the slow gup
> > path to get the physical page.
> > 
> > Instead of manually walking the page tables use __get_user_pages_fast
> > which does the same thing and it does not fall back to the slow gup
> > path.
> > 
> > This is largely inspired from kvm code. kvm uses __get_user_pages_fast
> > in hva_to_pfn_fast function which can run in an atomic context.
> > 
> > Cc: Ira Weiny <ira.weiny@...el.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Jérôme Glisse <jglisse@...hat.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Dimitri Sivanich <sivanich@....com>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: linux-kernel@...r.kernel.org
> > Cc: linux-mm@...ck.org
> > Signed-off-by: Bharath Vedartham <linux.bhar@...il.com>
> > ---
> >  drivers/misc/sgi-gru/grufault.c | 39 +++++----------------------------------
> >  1 file changed, 5 insertions(+), 34 deletions(-)
> > 
> > diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
> > index 75108d2..121c9a4 100644
> > --- a/drivers/misc/sgi-gru/grufault.c
> > +++ b/drivers/misc/sgi-gru/grufault.c
> > @@ -202,46 +202,17 @@ static int non_atomic_pte_lookup(struct vm_area_struct *vma,
> >  static int atomic_pte_lookup(struct vm_area_struct *vma, unsigned long vaddr,
> >  	int write, unsigned long *paddr, int *pageshift)
> >  {
> > -	pgd_t *pgdp;
> > -	p4d_t *p4dp;
> > -	pud_t *pudp;
> > -	pmd_t *pmdp;
> > -	pte_t pte;
> > -
> > -	pgdp = pgd_offset(vma->vm_mm, vaddr);
> > -	if (unlikely(pgd_none(*pgdp)))
> > -		goto err;
> > -
> > -	p4dp = p4d_offset(pgdp, vaddr);
> > -	if (unlikely(p4d_none(*p4dp)))
> > -		goto err;
> > -
> > -	pudp = pud_offset(p4dp, vaddr);
> > -	if (unlikely(pud_none(*pudp)))
> > -		goto err;
> > +	struct page *page;
> >  
> > -	pmdp = pmd_offset(pudp, vaddr);
> > -	if (unlikely(pmd_none(*pmdp)))
> > -		goto err;
> > -#ifdef CONFIG_X86_64
> > -	if (unlikely(pmd_large(*pmdp)))
> > -		pte = *(pte_t *) pmdp;
> > -	else
> > -#endif
> > -		pte = *pte_offset_kernel(pmdp, vaddr);
> > +	*pageshift = is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> >  
> > -	if (unlikely(!pte_present(pte) ||
> > -		     (write && (!pte_write(pte) || !pte_dirty(pte)))))
> > +	if (!__get_user_pages_fast(vaddr, 1, write, &page))
> >  		return 1;
> 
> Let's please use numeric, not boolean comparison, for the return value of 
> gup.
Alright then! I ll resubmit it!
> Also, optional: as long as you're there, atomic_pte_lookup() ought to
> either return a bool (true == success) or an errno, rather than a
> numeric zero or one.
That makes sense. But the code which uses atomic_pte_lookup uses the
return value of 1 for success and failure value of 0 in gru_vtop. That's
why I did not mess with the return values in this code. It would require
some change in the driver functionality which I am not ready to do :(
> Other than that, this looks like a good cleanup, I wonder how many
> open-coded gup implementations are floating around like this. 
I ll be on the lookout!
> thanks,
> -- 
> John Hubbard
> NVIDIA
> 
> >  
> > -	*paddr = pte_pfn(pte) << PAGE_SHIFT;
> > -
> > -	*pageshift = is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> > +	*paddr = page_to_phys(page);
> > +	put_user_page(page);
> >  
> >  	return 0;
> > -
> > -err:
> > -	return 1;
> >  }
> >  
> >  static int gru_vtop(struct gru_thread_state *gts, unsigned long vaddr,
> >