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From:   Bharath Vedartham <linux.bhar@gmail.com>
To:     arnd@arndb.de, sivanich@sgi.com, gregkh@linuxfoundation.org
Cc:     ira.weiny@intel.com, jhubbard@nvidia.com, jglisse@redhat.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Bharath Vedartham <linux.bhar@gmail.com>
Subject: [PATCH 3/3] sgi-gru: Use __get_user_pages_fast in atomic_pte_lookup
Date:   Sun, 21 Jul 2019 21:28:05 +0530
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*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;
 
-	*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,
-- 
2.7.4