Messages in this thread | | | From | (Eric W. Biederman) | Date | Wed, 09 May 2018 22:09:25 -0500 | Subject | Re: [PATCH] memcg: Replace mm->owner with mm->memcg |
| |
Oleg Nesterov <oleg@redhat.com> writes:
> On 05/07, Eric W. Biederman wrote: >> >> Oleg Nesterov <oleg@redhat.com> writes: >> >> > before your patch get_mem_cgroup_from_mm() looks at mm->owner == current >> > (in this case) and mem_cgroup_from_task() should return the correct memcg >> > even if execing task migrates after bprm_mm_init(). At least in the common >> > case when the old mm is not shared. >> > >> > After your patch the memory allocations in copy_strings() won't be accounted >> > correctly, bprm->mm->memcg is wrong if this task migrates. And iiuc your recent >> > "[PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm" >> > doesn't fix the problem. >> > >> > No? >> >> The patch does solve the issue. There should be nothing a userspace >> process can observe that should tell it where in the middle of exec >> such a migration happend so placing the migration at what from the >> kernel's perspective might be technically later should not be a problem. >> >> If it is a problem the issue is that there is a way to observe the >> difference. > > So. The task migrates from some MEMCG right after bprm_mm_init(). > > copy_strings() triggers OOM in MEMCG. This is quite possible, it can use a lot > of memory and that is why we have acct_arg_size() to make these allocations > visible to oom killer. > > task_in_mem_cgroup(MEMCG) returns false and oom killer has to kill another > innocent process in MEMCG. > > Does this look like a way to observe the difference?
Sort of.
I don't know how the memcg gets away without migrating charges when it migrates a process. With charges not being migrated I don't think this is observable.
That does look like a real issue however.
>> > Perhaps we can change get_mem_cgroup_from_mm() to use >> > mem_cgroup_from_css(current, memory_cgrp_id) if mm->memcg == NULL? >> >> Please God no. Having any unnecessary special case is just going to >> confuse people and cause bugs. > > To me the unnecessary special case is the new_mm->memcg which is used for > accounting but doesn't follow migration till exec_mmap(). But I won't > argue.
Eric
| |