mm/mempolicy.c: check range first in queue_pages_test_walk

Patch series "mm: Fix checking unmapped holes for mbind", v4.

This patchset fix checking unmapped holes for mbind().

First patch makes sure the vma been correctly tracked in .test_walk(),
so each time when .test_walk() is called, the neighborhood of two vma
is correct.

Current problem is that the !vma_migratable() check could cause return
immediately without update tracking to vma.

Second patch fix the inconsistent report of EFAULT when mbind() is
called for MPOL_DEFAULT and non MPOL_DEFAULT cases, so application do
not need to have workaround code to handle this special behavior.
Currently there are two problems, one is that the .test_walk() can not
know there is hole at tail side of range, because .test_walk() only
call for vma not for hole.  The other one is that mbind_range() checks
for hole at head side of range but do not consider the
MPOL_MF_DISCONTIG_OK flag as done in .test_walk().

This patch (of 2):

Checking unmapped hole and updating the previous vma must be handled
first, otherwise the unmapped hole could be calculated from a wrong
previous vma.

Several commits were relevant to this error:

 - commit 6f4576e368 ("mempolicy: apply page table walker on
   queue_pages_range()")

   This commit was correct, the VM_PFNMAP check was after updating
   previous vma

 - commit 48684a65b4 ("mm: pagewalk: fix misbehavior of
   walk_page_range for vma(VM_PFNMAP)")

   This commit added VM_PFNMAP check before updating previous vma. Then,
   there were two VM_PFNMAP check did same thing twice.

 - commit acda0c3340 ("mm/mempolicy.c: get rid of duplicated check for
   vma(VM_PFNMAP) in queue_page s_range()")

   This commit tried to fix the duplicated VM_PFNMAP check, but it
   wrongly removed the one which was after updating vma.

Link: http://lkml.kernel.org/r/1573218104-11021-2-git-send-email-lixinhai.lxh@gmail.com
Fixes: acda0c3340 (mm/mempolicy.c: get rid of duplicated check for vma(VM_PFNMAP) in queue_pages_range())
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: linux-man <linux-man@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -618,6 +618,16 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	unsigned long endvma = vma->vm_end;
 	unsigned long flags = qp->flags;
 
+	/* range check first */
+	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
+		if (!vma->vm_next && vma->vm_end < end)
+			return -EFAULT;
+		if (qp->prev && qp->prev->vm_end < vma->vm_start)
+			return -EFAULT;
+	}
+
+	qp->prev = vma;
+
 	/*
 	 * Need check MPOL_MF_STRICT to return -EIO if possible
 	 * regardless of vma_migratable
@@ -631,15 +641,6 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
 	if (vma->vm_start > start)
 		start = vma->vm_start;
 
-	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
-		if (!vma->vm_next && vma->vm_end < end)
-			return -EFAULT;
-		if (qp->prev && qp->prev->vm_end < vma->vm_start)
-			return -EFAULT;
-	}
-
-	qp->prev = vma;
-
 	if (flags & MPOL_MF_LAZY) {
 		/* Similar to task_numa_work, skip inaccessible VMAs */
 		if (!is_vm_hugetlb_page(vma) &&