Re: [PATCH -next v3 2/2] blk-throttle: fix io hung due to configuration updates

Hello,

On Fri, May 20, 2022 at 06:03:05PM +0200, Michal Koutný wrote:

quoted

Then io hung can be triggered by always submmiting new configuration
before the throttled bio is dispatched.

How big is this a problem actually? Is it only shooting oneself in the leg
or can there be a user who's privileged enough to modify throttling
configuration yet not privileged enough to justify the hung's
consequences (like some global FS locks).

So, the problem in itself is of the self-inflicted type and I'd prefer to
ignore it. Unfortunately, the kernel doesn't have the kind of isolation
where stalling out some aribtrary tasks is generally safe, especially not
blk-throtl as it doesn't handle bio_issue_as_root() and thus can have a
pretty severe priority inversions where IOs which can block system-wide
operations (e.g. memory reclaim) get trapped in a random cgroup.

Even ignoring that, the kernel in general assumes some forward progress from
everybody and when a part stalls it's relatively easy to spread to the rest
of the system, sometimes gradually, sometimes suddenly - e.g. if the stalled
IO was being performed while holding the mmap_sem, which isn't rare, then
anything which tries to read its proc cmdline will hang behind it.

So, we wanna avoid a situation where a non-priviledged user can cause
indefinite UNINTERRUPTIBLE sleeps to prevent local DoS attacks. I mean,
preventing local attacks is almost never fool proof but we don't want to
make it too easy at least.

Thanks.

-- 
tejun