On Tue, 3 Dec 2013, Johannes Weiner wrote: > > > Spin on which level? The whole point of this change was to not spin for > > > ever because the caller might sit on top of other locks which might > > > prevent somebody else to die although it has been killed. > > > > See my question about the non-memcg page allocator behavior below. > > No, please answer the question. > The question would be answered below, by having consistency in allocation and charging paths between both the page allocator and memcg. > > I'm not quite sure how significant of a point this is, though, because it > > depends on the caller doing the __GFP_NOFAIL allocations that allow the > > bypass. If you're doing > > > > for (i = 0; i < 1 << 20; i++) > > page[i] = alloc_page(GFP_NOFS | __GFP_NOFAIL); > > Hyperbole serves no one. > Since this bypasses all charges to the root memcg in oom conditions as a result of your patch, how do you ensure the "leakage" is contained to a small amount of memory? Are we currently just trusting the users of __GFP_NOFAIL that they aren't allocating a large amount of memory? > > I'm referring to the generic non-memcg page allocator behavior. Forget > > memcg for a moment. What is the behavior in the _page_allocator_ for > > GFP_NOFS | __GFP_NOFAIL? Do we spin forever if reclaim fails or do we > > bypas the per-zone min watermarks to allow it to allocate because "it > > needs to succeed, it may be holding filesystem locks"? > > > > It's already been acknowledged in this thread that no bypassing is done > > in the page allocator and it just spins. There's some handwaving saying > > that since the entire system is oom that there is a greater chance that > > memory will be freed by something else, but that's just handwaving and is > > certainly no guaranteed. > > Do you have another explanation of why this deadlock is not triggering > in the global case? It's pretty obvious that there is a deadlock that > can not be resolved unless some unrelated task intervenes, just read > __alloc_pages_slowpath(). > > But we had a concrete bug report for memcg where there was no other > task to intervene. One was stuck in the OOM killer waiting for the > victim to exit, the victim was stuck on locks that the killer held. > I believe the page allocator would be susceptible to the same deadlock if nothing else on the system can reclaim memory and that belief comes from code inspection that shows __GFP_NOFAIL is not guaranteed to ever succeed in the page allocator as their charges now are (with your patch) in memcg. I do not have an example of such an incident. > > So, my question again: why not bypass the per-zone min watermarks in the > > page allocator? > > I don't even know what your argument is supposed to be. The fact that > we don't do it in the page allocator means that there can't be a bug > in memcg? > I'm asking if we should allow GFP_NOFS | __GFP_NOFAIL allocations in the page allocator to bypass per-zone min watermarks after reclaim has failed since the oom killer cannot be called in such a context so that the page allocator is not susceptible to the same deadlock without a complete depletion of memory reserves? It's not an argument, it's a question. Relax. -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
