On 18.10.2021 12:04, Michal Hocko wrote: > On Mon 18-10-21 11:13:52, Vasily Averin wrote: > [...] >> How could this happen? >> >> User-space task inside the memcg-limited container generated a page fault, >> its handler do_user_addr_fault() called handle_mm_fault which could not >> allocate the page due to exceeding the memcg limit and returned VM_FAULT_OOM. >> Then do_user_addr_fault() called pagefault_out_of_memory() which executed >> out_of_memory() without set of memcg. >> >> Partially this problem depends on one of my recent patches, disabled unlimited >> memory allocation for dying tasks. However I think the problem can happen >> on non-killed tasks too, for example because of kmem limit. > > Could you be more specific on how this can happen without your patch? I > have to say I haven't realized this side effect when discussing it. We can reach obj_cgroup_charge_pages() for example via do_user_addr_fault handle_mm_fault __handle_mm_fault p4d_alloc __p4d_alloc p4d_alloc_one get_zeroed_page __get_free_pages alloc_pages __alloc_pages __memcg_kmem_charge_page obj_cgroup_charge_pages Here we call try_charge_memcg() that return success and approve the allocation, however then we hit into kmem limit and fail the allocation. If required I can try to search how try_charge_memcg() can reject page allocation of non-dying task too. > I will be honest that I am not really happy about pagefault_out_of_memory. > I have tried to remove it in the past. Without much success back then, > unfortunately[1]. > Maybe we should get rid of it finally. The OOM is always triggered from > inside the allocator where we have much more infromation about the > allocation context. A first step would be to skip pagefault_out_of_memory > for killed or exiting processes. I like this idea, however it may be not enough, at least in scenario described above. > [1] I do not have msg-id so I cannot provide a lore link but google > pointed me to https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1400402.html Thank you, I'll read this discussion. Vasily Averin
