Re: [PATCH 1/2] Revert "vmalloc: back off when the current task is killed"

[Johannes Weiner]

On Wed, Oct 04, 2017 at 03:32:45PM -0700, Andrew Morton wrote:
> On Wed, 4 Oct 2017 14:59:06 -0400 Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> 
> > This reverts commit 5d17a73a2ebeb8d1c6924b91e53ab2650fe86ffb and
> > commit 171012f561274784160f666f8398af8b42216e1f.
> > 
> > 5d17a73a2ebe ("vmalloc: back off when the current task is killed")
> > made all vmalloc allocations from a signal-killed task fail. We have
> > seen crashes in the tty driver from this, where a killed task exiting
> > tries to switch back to N_TTY, fails n_tty_open because of the vmalloc
> > failing, and later crashes when dereferencing tty->disc_data.
> > 
> > Arguably, relying on a vmalloc() call to succeed in order to properly
> > exit a task is not the most robust way of doing things. There will be
> > a follow-up patch to the tty code to fall back to the N_NULL ldisc.
> > 
> > But the justification to make that vmalloc() call fail like this isn't
> > convincing, either. The patch mentions an OOM victim exhausting the
> > memory reserves and thus deadlocking the machine. But the OOM killer
> > is only one, improbable source of fatal signals. It doesn't make sense
> > to fail allocations preemptively with plenty of memory in most cases.
> > 
> > The patch doesn't mention real-life instances where vmalloc sites
> > would exhaust memory, which makes it sound more like a theoretical
> > issue to begin with. But just in case, the OOM access to memory
> > reserves has been restricted on the allocator side in cd04ae1e2dc8
> > ("mm, oom: do not rely on TIF_MEMDIE for memory reserves access"),
> > which should take care of any theoretical concerns on that front.
> > 
> > Revert this patch, and the follow-up that suppresses the allocation
> > warnings when we fail the allocations due to a signal.
> 
> You don't think they should be backported into -stables?

Good point. For this one, it makes sense to CC stable, for 4.11 and
up. The second patch is more of a fortification against potential
future issues, and probably shouldn't go into stable.