*Georgi Nikolov* System Administrator www.icdsoft.com <http://www.icdsoft.com> On 08/01/2018 11:33 AM, Michal Hocko wrote: > On Wed 01-08-18 09:34:23, Vlastimil Babka wrote: >> On 07/31/2018 04:05 PM, Florian Westphal wrote: >>> Georgi Nikolov <gnikolov@xxxxxxxxxxx> 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@xxxxxxxxxxxxxx/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. Can we make this configurable - on/off switch or size above which to pass GFP_NORETRY. Probably hard coded based on amount of RAM is a good idea too. >> __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.
