Re: [PATCH v2] memcg: allow exiting tasks to write back data to swap

[Rik van Riel]

On Tue, 2025-01-14 at 17:46 +0100, Michal Hocko wrote:
> On Tue 14-01-25 11:09:55, Johannes Weiner wrote:
> 
> > 
> > We managed to extract a stack trace of the livelocked task:
> > 
> > obj_cgroup_may_swap
> > zswap_store
> > swap_writepage
> > shrink_folio_list
> > shrink_lruvec
> > shrink_node
> > do_try_to_free_pages
> > try_to_free_mem_cgroup_pages
> 
> OK, so this is the reclaim path and it fails due to reasons you
> mention
> below. This will retry several times until it hits mem_cgroup_oom
> which
> will bail in mem_cgroup_out_of_memory because of task_is_dying
> (returns
> true) and retry the charge + reclaim (as the oom killer hasn't done
> anything) with passed_oom = true this time and eventually got to
> nomem
> path and returns ENOMEM. This should propaged -ENOMEM down the path
> 
> > charge_memcg
> > mem_cgroup_swapin_charge_folio
> > __read_swap_cache_async
> > swapin_readahead
> > do_swap_page
> > handle_mm_fault
> > do_user_addr_fault
> > exc_page_fault
> > asm_exc_page_fault
> > __get_user
> 
> All the way here and return the failure to futex_cleanup which
> doesn't
> retry __get_user on the failure AFAICS (exit_robust_list). But I
> might
> be missing something, it's been quite some time since I've looked
> into
> futex code.

Can you explain how -ENOMEM would get propagated down
past the page fault handler?

This isn't get_user_pages(), which can just pass
-ENOMEM on to the caller.

If there is code to pass -ENOMEM on past the page
fault exception handler, I have not been able to
find it. How does this work?

> 
> > futex_cleanup
> > fuxtex_exit_release
> > do_exit
> > do_group_exit
> > get_signal
> > arch_do_signal_or_restart
> > exit_to_user_mode_prepare
> > syscall_exit_to_user_mode
> > do_syscall
> > entry_SYSCALL_64
> > syscall
> > 
> > Both memory.max and memory.zswap.max are hit. I don't see how this
> > could ever make forward progress - the futex fault will retry until
> > it
> > succeeds.
> 
> I must be missing something but I do not see the retry, could you
> point
> me where this is happening please?
> 

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