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" <[email protected]>
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 <[email protected]>
Fixes: 6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")
Reported-by: Will Deacon <[email protected]>
---
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
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