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
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 <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
---
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;
}
--- On Sat, 3/19/11, Hugh Dickins <[email protected]> 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
On Sun, 20 Mar 2011, Kenny Simpson wrote:
> --- On Sat, 3/19/11, Hugh Dickins <[email protected]> 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: [email protected]
to [PATCH] shmem: let shared anonymous be nonlinear again
Thanks,
Hugh