BACKPORT: FROMLIST: mm: enable speculative fault handling for supported file types.

Introduce vma_can_speculate(), which allows speculative handling for
VMAs mapping supported file types.

From do_handle_mm_fault(), speculative handling will follow through
__handle_mm_fault(), handle_pte_fault() and do_fault().

At this point, we expect speculative faults to continue through one of:
- do_read_fault(), fully implemented;
- do_cow_fault(), which might abort if missing anon vmas,
- do_shared_fault(), not implemented yet
  (would require ->page_mkwrite() changes).

vma_can_speculate() provides an early abort for the do_shared_fault() case,
limiting the time spent on trying that unimplemented case.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-31-michel@lespinasse.org/

Conflicts:
    include/linux/vm_event_item.h
    mm/vmstat.c

1. SPF_ATTEMPT_FILE is taken from https://lore.kernel.org/all/20210407014502.24091-36-michel@lespinasse.org/
since the patch posted upstream at the time had a different structure
with stats for anonymouse and file-backed pagefaults introduced in a
separate patch.

Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I3a28af63b41b649f02f8b73d53f6494ad114ee5a

include/linux/mm.h

@@ -722,6 +722,20 @@ static inline bool vma_is_accessible(struct vm_area_struct *vma)
 	return vma->vm_flags & VM_ACCESS_FLAGS;
 }
 
+static inline bool vma_can_speculate(struct vm_area_struct *vma,
+		unsigned int flags)
+{
+	if (vma_is_anonymous(vma))
+		return true;
+	if (!vma->vm_ops->speculative)
+		return false;
+	if (!(flags & FAULT_FLAG_WRITE))
+		return true;
+	if (!(vma->vm_flags & VM_SHARED))
+		return true;
+	return false;
+}
+
 #ifdef CONFIG_SHMEM
 /*
  * The vma_is_shmem is not inline because it is used only by slow