Re: [PATCH] mm, percpu: do not consider sleepable allocations atomic

Michal Hocko
2025-03-05T15:10:20Z

Sorry, I have missed follow ups here.

On Fri 21-02-25 10:48:28, Vlastimil Babka wrote:
> On 2/21/25 03:36, Dennis Zhou wrote:
> > I've thought about this in the back of my head for the past few weeks. I
> > think I have 2 questions about this change.
> >
> > 1. Back to what TJ said earlier about probing. I feel like GFP_KERNEL
> > allocations should be okay because that more or less is control plane
> > time? I'm not sure dropping PR_SET_IO_FLUSHER is all that big of a
> > work around?
>
> This solves the iscsid case but not other cases, where GFP_KERNEL
> allocations are fundamentally impossible.

Agreed

>
> > 2. This change breaks the feedback loop as we discussed above.
> > Historically we've targeted 2-4 free pages worth of percpu memory.
> > This is done by kicking the percpu work off. That does GFP_KERNEL
> > allocations and if that requires reclaim then it goes and does it.
> > However, now we're saying kswapd is going to work in parallel while
> > we try to get pages in the worker thread.
> >
> > Given you're more versed in the reclaim side. I presume it must be
> > pretty bad if we're failing to get order-0 pages even if we have
> > NOFS/NOIO set?
>
> IMHO yes, so I don't think we need to pre-emptively fear that situation that
> much. OTOH in the current state, depleting pcpu's atomic reserves and
> failing pcpu_alloc due to not being allowed to take the mutex can happen
> easily and even if there's plenty of free memory.

Agreed

> > My feeling is that we should add back some knowledge of the
> > dependency so if the worker fails to get pages, it doesn't reschedule
> > immediately. Maybe it's as simple as adding a sleep in the worker or
> > playing with delayed work...
>
> I think if we wanted things to be more robust (and perhaps there's no need
> to, see above), the best way would be to make the worker preallocate with
> GFP_KERNEL outside of pcpu_alloc_mutex.

Yes this would work as it would break the lock chain dependency.

> I assume it's probably not easy to
> implement as page table allocations are involved in the process and we don't
> have a way to supply preallocated memory for those.

Why would this be a concern if the allocation is done outside of the
lock?
--
Michal Hocko
SUSE Labs