Fix ZERO_PAGE breakage with vmware

Commit 89f5b7da2a6bad2e84670422ab8192382a5aeb9f ("Reinstate ZERO_PAGE
optimization in 'get_user_pages()' and fix XIP") broke vmware, as
reported by Jeff Chua:

  "This broke vmware 6.0.4.
   Jun 22 14:53:03.845: vmx| NOT_IMPLEMENTED
   /build/mts/release/bora-93057/bora/vmx/main/vmmonPosix.c:774"

and the reason seems to be that there's an old bug in how we handle do
FOLL_ANON on VM_SHARED areas in get_user_pages(), but since it only
triggered if the whole page table was missing, nobody had apparently hit
it before.

The recent changes to 'follow_page()' made the FOLL_ANON logic trigger
not just for whole missing page tables, but for individual pages as
well, and exposed this problem.

This fixes it by making the test for when FOLL_ANON is used more
careful, and also makes the code easier to read and understand by moving
the logic to a separate inline function.

Reported-and-tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 9aefaae46858..423e0e7c2f73 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1045,6 +1045,26 @@ no_page_table:
         return page;
 }
 
+/* Can we do the FOLL_ANON optimization? */
+static inline int use_zero_page(struct vm_area_struct *vma)
+{
+        /*
+         * We don't want to optimize FOLL_ANON for make_pages_present()
+         * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
+         * we want to get the page from the page tables to make sure
+         * that we serialize and update with any other user of that
+         * mapping.
+         */
+        if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
+                return 0;
+        /*
+         * And if we have a fault or a nopfn routine, it's not an
+         * anonymous region.
+         */
+        return !vma->vm_ops ||
+                (!vma->vm_ops->fault && !vma->vm_ops->nopfn);
+}
+
 int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                 unsigned long start, int len, int write, int force,
                 struct page **pages, struct vm_area_struct **vmas)
@@ -1119,8 +1139,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                 foll_flags = FOLL_TOUCH;
                 if (pages)
                         foll_flags |= FOLL_GET;
-                if (!write && !(vma->vm_flags & VM_LOCKED) &&
-                    (!vma->vm_ops || !vma->vm_ops->fault))
+                if (!write && use_zero_page(vma))
                         foll_flags |= FOLL_ANON;
 
                 do {