mm: handle_pte_fault() use pte_offset_map_rw_nolock()

In handle_pte_fault(), we may modify the vmf->pte after acquiring the
vmf->ptl, so convert it to using pte_offset_map_rw_nolock().  But since we
will do the pte_same() check, so there is no need to get pmdval to do
pmd_same() check, just pass a dummy variable to it.

Link: https://lkml.kernel.org/r/af8d694853b44c5a6018403ae435440e275854c7.1727332572.git.zhengqi.arch@bytedance.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index c8d5d040d6ab113e896be7ebade6a0557090eda1..ce5cd8d4c4011a42574cf6e05c0de5cf6e11874b 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5742,14 +5742,24 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
                vmf->pte = NULL;
                vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
        } else {
+               pmd_t dummy_pmdval;
+
                /*
                 * A regular pmd is established and it can't morph into a huge
                 * pmd by anon khugepaged, since that takes mmap_lock in write
                 * mode; but shmem or file collapse to THP could still morph
                 * it into a huge pmd: just retry later if so.
+                *
+                * Use the maywrite version to indicate that vmf->pte may be
+                * modified, but since we will use pte_same() to detect the
+                * change of the !pte_none() entry, there is no need to recheck
+                * the pmdval. Here we chooes to pass a dummy variable instead
+                * of NULL, which helps new user think about why this place is
+                * special.
                 */
-               vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-                                                vmf->address, &vmf->ptl);
+               vmf->pte = pte_offset_map_rw_nolock(vmf->vma->vm_mm, vmf->pmd,
+                                                   vmf->address, &dummy_pmdval,
+                                                   &vmf->ptl);
                if (unlikely(!vmf->pte))
                        return 0;
                vmf->orig_pte = ptep_get_lockless(vmf->pte);