mm: introduce follow_pte()

A generic readonly page table lookup helper to map an address space and an
address from it to a pte.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Hellwig <hch@infradead.org>
Acked-by: Magnus Damm <magnus.damm@gmail.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 0 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 891bad0613f..7135d6b2599 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3103,6 +3103,43 @@ int in_gate_area_no_task(unsigned long addr)
 
 #endif  /* __HAVE_ARCH_GATE_AREA */
 
+static int follow_pte(struct mm_struct *mm, unsigned long address,
+                pte_t **ptepp, spinlock_t **ptlp)
+{
+        pgd_t *pgd;
+        pud_t *pud;
+        pmd_t *pmd;
+        pte_t *ptep;
+
+        pgd = pgd_offset(mm, address);
+        if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
+                goto out;
+
+        pud = pud_offset(pgd, address);
+        if (pud_none(*pud) || unlikely(pud_bad(*pud)))
+                goto out;
+
+        pmd = pmd_offset(pud, address);
+        if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
+                goto out;
+
+        /* We cannot handle huge page PFN maps. Luckily they don't exist. */
+        if (pmd_huge(*pmd))
+                goto out;
+
+        ptep = pte_offset_map_lock(mm, pmd, address, ptlp);
+        if (!ptep)
+                goto out;
+        if (!pte_present(*ptep))
+                goto unlock;
+        *ptepp = ptep;
+        return 0;
+unlock:
+        pte_unmap_unlock(ptep, *ptlp);
+out:
+        return -EINVAL;
+}
+
 #ifdef CONFIG_HAVE_IOREMAP_PROT
 int follow_phys(struct vm_area_struct *vma,
                 unsigned long address, unsigned int flags,