linux-kernel - Re: Occasional hung with UM after enable VMAP

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Date:   Wed, 5 Jan 2022 09:58:36 -0300
From:   Walter Lozano <walter.lozano@...labora.com>
To:     Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Sjoerd Simons <sjoerd.simons@...labora.com>,
        ritesh sarraf <ritesh.sarraf@...labora.com>
Subject: Re: Occasional hung with UM after enable VMAP_STACK

Hi Johannes,

On 1/4/22 16:49, Johannes Berg wrote:
> On Tue, 2022-01-04 at 16:26 -0300, Walter Lozano wrote:
>> Thank you for your quick response. The Debian configuration on package
>> user-mode-linux have these settings
>>
>> CONFIG_HAVE_ARCH_VMAP_STACK=y
>> CONFIG_VMAP_STACK=y
> OK, so it actually _is_ enabled.
>
>> as you can see in [1]. I did run some tests disabling those settings,
>> which passed without any hung.
>>
>> Unfortunately the "occasional" behavior makes this issue a bit tricky to
>> debug.
>>
> Right.
>
> Hm. I've been running our tests with it for about three months and
> haven't observed any hangs, but I guess that doesn't mean much.

The issue is very rare at least in my setup, I had to prepare a test 
environment to stress the system in order to be able to reproduce it 
more or less consistently. I will continue investigating and share any 
useful information.

> To be honest, I have no particular reason to even want it, other than
> that it catches accidental DMA from stack more easily ... so I guess if
> we can't find anything, we might as well revert it.

I hope we will be able to find the root cause of the issue.

> Feels like it _should_ work though, since it's just a different location
> for the stack.


I see. Thank you again for your quick reply, I will keep you updated.

Regards,

Walter

-- 
Walter Lozano
Collabora Ltd.