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Subject: Re: [PATCH 3/3] sgi-gru: Use __get_user_pages_fast in
 atomic_pte_lookup
To:     Bharath Vedartham <linux.bhar@gmail.com>, <arnd@arndb.de>,
        <sivanich@sgi.com>, <gregkh@linuxfoundation.org>
CC:     <ira.weiny@intel.com>, <jglisse@redhat.com>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>
References: <1563724685-6540-1-git-send-email-linux.bhar@gmail.com>
 <1563724685-6540-4-git-send-email-linux.bhar@gmail.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <c508330d-a5d0-fba3-9dd0-eb820a96ee09@nvidia.com>
Date:   Sun, 21 Jul 2019 19:32:36 -0700
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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.
>=20
> 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.
>=20
> 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.
>=20
> 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.
>=20
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: J=C3=A9r=C3=B4me 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(-)
>=20
> diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufa=
ult.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_str=
uct *vma,
>  static int atomic_pte_lookup(struct vm_area_struct *vma, unsigned long v=
addr,
>  	int write, unsigned long *paddr, int *pageshift)
>  {
> -	pgd_t *pgdp;
> -	p4d_t *p4dp;
> -	pud_t *pudp;
> -	pmd_t *pmdp;
> -	pte_t pte;
> -
> -	pgdp =3D pgd_offset(vma->vm_mm, vaddr);
> -	if (unlikely(pgd_none(*pgdp)))
> -		goto err;
> -
> -	p4dp =3D p4d_offset(pgdp, vaddr);
> -	if (unlikely(p4d_none(*p4dp)))
> -		goto err;
> -
> -	pudp =3D pud_offset(p4dp, vaddr);
> -	if (unlikely(pud_none(*pudp)))
> -		goto err;
> +	struct page *page;
> =20
> -	pmdp =3D pmd_offset(pudp, vaddr);
> -	if (unlikely(pmd_none(*pmdp)))
> -		goto err;
> -#ifdef CONFIG_X86_64
> -	if (unlikely(pmd_large(*pmdp)))
> -		pte =3D *(pte_t *) pmdp;
> -	else
> -#endif
> -		pte =3D *pte_offset_kernel(pmdp, vaddr);
> +	*pageshift =3D is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> =20
> -	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=20
gup.

Also, optional: as long as you're there, atomic_pte_lookup() ought to
either return a bool (true =3D=3D success) or an errno, rather than a
numeric zero or one.

Other than that, this looks like a good cleanup, I wonder how many
open-coded gup implementations are floating around like this.=20

thanks,
--=20
John Hubbard
NVIDIA

> =20
> -	*paddr =3D pte_pfn(pte) << PAGE_SHIFT;
> -
> -	*pageshift =3D is_vm_hugetlb_page(vma) ? HPAGE_SHIFT : PAGE_SHIFT;
> +	*paddr =3D page_to_phys(page);
> +	put_user_page(page);
> =20
>  	return 0;
> -
> -err:
> -	return 1;
>  }
> =20
>  static int gru_vtop(struct gru_thread_state *gts, unsigned long vaddr,
>=20