Re: [PATCH v5 1/5] sysfs: Add attribute info for /sys/devices/.../coredump_disabled

[Arend van Spriel]

On August 12, 2022 8:09:59 AM Greg Kroah-Hartman 
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, Aug 11, 2022 at 04:21:54PM -0700, Manish Mandlik wrote:
> > On Wed, Aug 10, 2022 at 9:21 AM Greg Kroah-Hartman <
> > gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > On Wed, Aug 10, 2022 at 06:03:37PM +0200, Johannes Berg wrote:
> > > > On Wed, 2022-08-10 at 09:00 -0700, Manish Mandlik wrote:
> > > > > This patch adds the specification for
> > > /sys/devices/.../coredump_disabled
> > > > > attribute which allows the userspace to enable/disable devcoredump for
> > > a
> > > > > particular device and drivers can use it to enable/disable
> > > functionality
> > > > > accordingly. It is available when the CONFIG_DEV_COREDUMP is enabled
> > > and
> > > > > driver has implemented the .coredump() callback.
> > > >
> > > > It would be nice to say _why_? What problem does this solve? You could
> > > > just create the dump and discard it, instead, for example?
> > >
> > > Agreed, I do not understand the need for this at all.
> >
> > The existing /sys/class/devcoredump/disabled (devcd) switch has two
> > limitations - it disables dev_coredump for everyone who's using it;
>
> Which is good and is the design of the thing.
>
> > and
> > drivers don't have visibility if devcd is disabled or not, so, the
> > dev_coredump API simply lets drivers collect the coredump from a device but
> > then later discards it if devcd is disabled.
>
> Why would a driver care?
>
> > Now that there are more subsystems using the base dev_coredump API, having
> > a granular control will make it easier to selectively disable dev_coredump
> > only for a particular device. For ChromeOS, this is useful to allow drivers
> > to develop coredump functionality and deploy it without affecting other
> > drivers with stable devcoredump implementations (example, we've had some
> > devcoredumps that take 12s to run and we only want to enable it on test
> > builds because it has lots of PII). The drivers can use this flag to
> > refrain from collecting or triggering coredump when undesirable.
>
> This feels odd.  You have various out-of-tree drivers that take too long
> when they crash to make a dump and it causes some unknown issue
> elsewhere?

If you have drivers taking 12s for coredump you could/should consider doing 
it asynchronous, eg. schedule a worker for it. The coredump callback has 
void return type so it would be fairly easy.

Regards,
Arend