RE: LTP regressions due to 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Will Deacon wrote:
> Hi Kirill,
> 
> Your patch 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")
> causes some mmap regressions in LTP, which appears to use a MAP_PRIVATE
> mmap of /dev/zero as a way to get anonymous pages in some of its tests
> (specifically mmap10 [1]).
> 
> Dead simple reproducer below. Is this change in behaviour intentional?

Ouch. Of couse it's a bug.

Fix is below. I don't really like it, but I cannot find any better
solution.

>From 97be4458fd63758b0c233e528bf952d1cd26428e Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Mon, 14 Sep 2015 14:44:32 +0300
Subject: [PATCH] mm: fix mmap(MAP_PRIVATE) on /dev/zero

In attempt to make mm less fragile I've screwed up setting up anonymous
mappings by mmap() on /dev/zero.

Here's the fix.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")
Reported-by: Will Deacon <will.deacon@arm.com>
---
 drivers/char/mem.c | 2 +-
 include/linux/mm.h | 1 +
 mm/mmap.c          | 6 ++++--
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 6b1721f978c2..c8fe3af4de29 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -651,7 +651,7 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
 	return written;
 }
 
-static int mmap_zero(struct file *file, struct vm_area_struct *vma)
+int mmap_zero(struct file *file, struct vm_area_struct *vma)
 {
 #ifndef CONFIG_MMU
 	return -ENOSYS;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 91c08f6f0dc9..5e152e9588ec 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1066,6 +1066,7 @@ extern void pagefault_out_of_memory(void);
 extern void show_free_areas(unsigned int flags);
 extern bool skip_free_areas_node(unsigned int flags, int nid);
 
+int mmap_zero(struct file *file, struct vm_area_struct *vma);
 int shmem_zero_setup(struct vm_area_struct *);
 #ifdef CONFIG_SHMEM
 bool shmem_mapping(struct address_space *mapping);
diff --git a/mm/mmap.c b/mm/mmap.c
index 971dd2cb77d2..7960fd206a2f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -612,7 +612,9 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
 void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
 		struct rb_node **rb_link, struct rb_node *rb_parent)
 {
-	WARN_ONCE(vma->vm_file && !vma->vm_ops, "missing vma->vm_ops");
+	WARN_ONCE(vma->vm_file && !vma->vm_ops &&
+			vma->vm_file->f_op->mmap != mmap_zero,
+			"missing vma->vm_ops");
 
 	/* Update tracking information for the gap following the new vma. */
 	if (vma->vm_next)
@@ -1639,7 +1641,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		WARN_ON_ONCE(addr != vma->vm_start);
 
 		/* All file mapping must have ->vm_ops set */
-		if (!vma->vm_ops) {
+		if (!vma->vm_ops && file->f_op->mmap != &mmap_zero) {
 			static const struct vm_operations_struct dummy_ops = {};
 			vma->vm_ops = &dummy_ops;
 		}
-- 
2.5.1