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

[Michal Hocko]

On Thu 05-10-17 19:36:17, Tetsuo Handa wrote:
> On 2017/10/05 16:57, Michal Hocko wrote:
> > On Wed 04-10-17 19:18:21, Johannes Weiner wrote:
> >> On Wed, Oct 04, 2017 at 03:32:45PM -0700, Andrew Morton wrote:
> > [...]
> >>> 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.
> > 
> > I am not against. It is true that the memory reserves depletion fix was
> > theoretical because I haven't seen any real life bug. I would argue that
> > the more robust allocation failure behavior is a stable candidate as
> > well, though, because the allocation can fail regardless of the vmalloc
> > revert. It is less likely but still possible.
> > 
> 
> I don't want this patch backported. If you want to backport,
> "s/fatal_signal_pending/tsk_is_oom_victim/" is the safer way.
> 
> On 2017/10/04 17:33, Michal Hocko wrote:
> > Now that we have cd04ae1e2dc8 ("mm, oom: do not rely on TIF_MEMDIE for
> > memory reserves access") the risk of the memory depletion is much
> > smaller so reverting the above commit should be acceptable. 
> 
> Are you aware that stable kernels do not have cd04ae1e2dc8 ?

yes

> We added fatal_signal_pending() check inside read()/write() loop
> because one read()/write() request could consume 2GB of kernel memory.

yes, because this is easily trigerable by userspace.

> What if there is a kernel module which uses vmalloc(1GB) from some
> ioctl() for legitimate reason? You are going to allow such vmalloc()
> calls to deplete memory reserves completely.

Do you have any specific example in mind? If yes we can handle it.
-- 
Michal Hocko
SUSE Labs