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

[Rosen Penev]

On Sun, May 6, 2018 at 10:08 AM, Rosen Penev <rosenp at gmail.com> wrote:
> 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
Disregard that. That commit should have no impact.

I just tested it on an Archer C7v4 and the issue is present as well.
This was probably causing data corruption for me when I used an
external hard drive...

Strange that sometimes it works and sometimes not.

Did you encounter this issue with kernel 4.9? For me, 4.4 caused no
data corruption on my external hard drive.

There seems to be a pattern with kernel 4.9 breaking various MIPS devices...
>> --
>> Regards
>>
>> Daniel Danzberger
>> embeDD GmbH, Alter Postplatz 2, CH-6370 Stans
>>
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