On Mon, Sep 14, 2015 at 12:57:59PM +0100, Kirill A. Shutemov wrote:
> Will Deacon wrote:
> > 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.
Brill, thanks for the quick response! I agree that the fix isn't very
nice. Maybe moving the mmap_zero test into a helper would make it more
palatable?
> 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]>
FWIW:
Tested-by: Will Deacon <[email protected]>
Cheers,
Will
> ---
> 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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