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SubjectRe: [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@intel.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Jérôme Glisse <jglisse@redhat.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Dimitri Sivanich <sivanich@sgi.com>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: linux-mm@kvack.org
> > Signed-off-by: Bharath Vedartham <linux.bhar@gmail.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,
> >

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