Re: [Bug 200651] New: cgroups iptables-restor: vmalloc: allocation failure

[Michal Hocko <mhocko@kernel.org>]

On Wed 01-08-18 09:34:23, Vlastimil Babka wrote:
> On 07/31/2018 04:05 PM, Florian Westphal wrote:
> > Georgi Nikolov <gnikolov@icdsoft.com> wrote:
> >>> No, I think that's rather for the netfilter folks to decide. However, it
> >>> seems there has been the debate already [1] and it was not found. The
> >>> conclusion was that __GFP_NORETRY worked fine before, so it should work
> >>> again after it's added back. But now we know that it doesn't...
> >>>
> >>> [1] https://lore.kernel.org/lkml/20180130140104.GE21609@dhcp22.suse.cz/T/#u
> >>
> >> Yes i see. I will add Florian Westphal to CC list. netfilter-devel is
> >> already in this list so probably have to wait for their opinion.
> > 
> > It hasn't changed, I think having OOM killer zap random processes
> > just because userspace wants to import large iptables ruleset is not a
> > good idea.
> 
> If we denied the allocation instead of OOM (e.g. by using
> __GFP_RETRY_MAYFAIL), a slightly smaller one may succeed, still leaving
> the system without much memory, so it will invoke OOM killer sooner or
> later anyway.
> 
> I don't see any silver-bullet solution, unfortunately. If this can be
> abused by (multiple) namespaces, then they have to be contained by
> kmemcg as that's the generic mechanism intended for this. Then we could
> use the __GFP_RETRY_MAYFAIL.
> The only limit we could impose to outright deny the allocation (to
> prevent obvious bugs/admin mistakes or abuses) could be based on the
> amount of RAM, as was suggested in the old thread.
> 
> __GFP_NORETRY might look like a good match at first sight as that stops
> allocating when "reclaim becomes hard" which means the system is still
> relatively far from OOM. But it's not reliable in principle, and as this
> bug report shows. That's fine when __GFP_NORETRY is used for optimistic
> allocations that have some other fallback (e.g. huge page with fallback
> to base page), but far from ideal when failure means returning -ENOMEM
> to userspace.

I absolutely agree. The whole __GFP_NORETRY is quite dubious TBH. I have
used it to get the original behavior because the change wasn't really
intended to make functional changes. But consideg ring this requires
higher privileges then I fail to see where the distrust comes from. If
this is really about untrusted root in a namespace then the proper way
is to use __GFP_ACCOUNT and limit that via kmemc.

__GFP_NORETRY can fail really easily if the kswapd doesn't keep the pace
with the allocations which might be completely unrelated to this
particular request.
-- 
Michal Hocko
SUSE Labs