Re: remap_file_pages doesn't like MAP_ANONYMOUS (but used to?)

[Hugh Dickins]

On Wed, 16 Mar 2011, Kenny Simpson wrote:
> Hello,
>   In older kernels (2.6.15-2.6.18?) I was able to make an anonymous mapping and remap part of it back over itself to create a circular buffer:
>   p = mmap(0, sz, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0)
>   remap_file_pages(p + sz/2, sz/2, 0/*prot*/, 0/*offset*/, 0/*flags*/)
> 
> I find the same method fails with EINVAL on the remap_file_pages call in 2.6.35/38.  Making a dummy file or shared memory object works, but seems a bit more messy.
> 
> Why drop support for ANONYMOUS?  Is there another way to get the same effect?

Thanks for reporting.  It was a simple oversight: and until you came
along, nobody noticed.  Here's the patch, I'll pass it upstream shortly.

[PATCH] shmem: let shared anonymous be nonlinear again

Up to 2.6.22, you could use remap_file_pages(2) on a tmpfs file or a
shared mapping of /dev/zero or a shared anonymous mapping.  In 2.6.23
we disabled it by default, but set VM_CAN_NONLINEAR to enable it on
safe mappings.  We made sure to set it in shmem_mmap() for tmpfs files,
but missed it in shmem_zero_setup() for the others.  Fix that at last.

Reported-by: Kenny Simpson <theonetruekenny@xxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/shmem.c |    1 +
 1 file changed, 1 insertion(+)

--- 2.6.38/mm/shmem.c	2011-03-14 18:20:32.000000000 -0700
+++ linux/mm/shmem.c	2011-03-19 15:09:26.000000000 -0700
@@ -2791,5 +2791,6 @@ int shmem_zero_setup(struct vm_area_stru
 		fput(vma->vm_file);
 	vma->vm_file = file;
 	vma->vm_ops = &shmem_vm_ops;
+	vma->vm_flags |= VM_CAN_NONLINEAR;
 	return 0;
 }