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

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

[Kenny Simpson]

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,
-Kenny



      

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

[Hugh Dickins]

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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@yahoo.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 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;
 }

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

[Kenny Simpson]

--- On Sat, 3/19/11, Hugh Dickins <hughd@google.com> wrote:
> 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.

Since this bug has existed for a while, could it be pushed back to the various stable/longterm branches as well? (as they all are affected)

Many thanks on the quick response!  Glad it was a simple oversight with an easy and obvious fix.

thanks again!
-Kenny



      

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

[Hugh Dickins]

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On Sun, 20 Mar 2011, Kenny Simpson wrote:
> --- On Sat, 3/19/11, Hugh Dickins <hughd@google.com> wrote:
> > 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.
> 
> Since this bug has existed for a while, could it be pushed back to the various stable/longterm branches as well? (as they all are affected)
> 
> Many thanks on the quick response!  Glad it was a simple oversight with an easy and obvious fix.
> 
> thanks again!
> -Kenny

It's a rare pleasure to be presented with such a straightforward bug!

stable/longterm: well, I don't mind asking akpm to add a Cc: stable,
but don't be surprised if he prefers not: although it was a regression
back in 2.6.23, it's not a security issue, and the lack of interest
between then and now doesn't make a strong argument for it.

Andrew, if you're so inclined, please add a Cc: stable@kernel.org
to [PATCH] shmem: let shared anonymous be nonlinear again

Thanks,
Hugh