[LEDE-DEV] MMAP memory out of sync on AR71xx Rambutan (8devices) board.

[Rosen Penev]

On Sun, May 6, 2018 at 3:52 AM, Daniel Danzberger <daniel at dd-wrt.com> wrote:
> MMAP'ed memory that has been allocated via 'get_zeroed_page(GFP_KERNEL)' or
> 'vmalloc()' doesn't always contain the same data when accessed from userspace.
>
> This means all userspace programs using mmap to access kernel memory aren't
> always working properly on the Rambutan board. I am currently testing if other
> ar71xx devices are affected as well.
>
> I first noticed this when using ALSA's mmap api to capture audio.
>
> Here is the feed for a kmod + userpace util to reproduce the issue:
> git at github.com:dddaniel/mmaptest.git
>
> The kernel module simply allocates a page and initializes it with 0xff.
> The userspace application then mmap's and reads this page 10 times with a 500ms
> delay an checks if all data is 0xff.
>
> ---
> root at OpenWrt:/# mmaptest-user
> mmap addr: 0x77a04000
> [  760.464968] mmap page 7573000 at va 87573000
> check memory ...FAIL (at byte 0)
> check memory ...FAIL (at byte 96)
> check memory ...FAIL (at byte 96)
> check memory ...FAIL (at byte 96)
> check memory ...FAIL (at byte 128)
> ---
>
> I have no idea whats causing it. Does anybody have a hint on how to fix this ?
>
Try reverting https://github.com/torvalds/linux/commit/c00ab4896ed5f7d89af6f90b809e2c0197c6d170
> --
> Regards
>
> Daniel Danzberger
> embeDD GmbH, Alter Postplatz 2, CH-6370 Stans
>
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