Re: [PATCH v2 0/6] mm/devm_memremap_pages: Fix page release race

[Dan Williams]

On Thu, May 16, 2019 at 9:45 AM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
>
> Hi,
>
> I'm able to reproduce the panic below by running two sets of ndctl
> commands that actually serve legitimate purpose in parallel (unlike
> the brute force experiment earlier), each set in a indefinite loop.
> This time it takes about an hour to panic.  But I gather the cause
> is probably the same: I've overlapped ndctl commands on the same
> region.
>
> Could we add a check in nd_ioctl(), such that if there is
> an ongoing ndctl command on a region, subsequent ndctl request
> will fail immediately with something to the effect of EAGAIN?
> The rationale being that kernel should protect itself against
> user mistakes.

We do already have locking in the driver to prevent configuration
collisions. The problem looks to be broken assumptions about running
the device unregistration path in a separate thread outside the lock.
I suspect it may be incorrect assumptions about the userspace
visibility of the device relative to teardown actions. To be clear
this isn't the nd_ioctl() path this is the sysfs path.


> Also, sensing the subject fix is for a different problem, and has been
> verified, I'm happy to see it in upstream, so we have a better
> code base to digger deeper in terms of how the destructive ndctl
> commands interacts to typical mission critical applications, include
> but not limited to rdma.

Right, the crash signature you are seeing looks unrelated to the issue
being address in these patches which is device-teardown racing active
page pins. I'll start the investigation on the crash signature, but
again I don't think it reads on this fix series.