Re: [PATCH 1/2] mm/percpu: Preserve NOFS/NOIO scope during chunk create and populate

[Michal Hocko <mhocko@suse.com>]

On Tue 02-06-26 11:03:15, Kaitao Cheng wrote:
> 
> 
> 在 2026/6/1 23:45, Michal Hocko 写道:
> > On Mon 01-06-26 10:27:53, Kaitao Cheng wrote:
> >> However, if we revert 9a5b183941b, it seems that all of these issues would
> >> be resolved. The only downside is that the failure rate of pcpu_alloc_noprof()
> >> allocations may increase, which might be acceptable.
> > 
> > That has practical impact on some versions of iscsid which do not have
> > PR_SET_IO_FLUSHER. And maybe some more so I would rather not revert
> > based on a theoretical concerns which I believe is the case here. 
> > 
> 
> Based on the previous discussion, I think we have a way to address most
> of the concurrency issues around percpu allocation.
> 
> However, there still seems to be one remaining case that I do not yet
> have a good way to solve. For example:
> 
> Thread A calls pcpu_alloc_noprof() with GFP_KERNEL and takes
> pcpu_alloc_mutex. Since the internal allocation is not constrained by
> NOFS, it may enter FS reclaim while still holding pcpu_alloc_mutex,
> creating a dependency like:
> 
> pcpu_alloc_mutex -> fs_reclaim -> FS lock
> At the same time, Thread B may already hold an FS lock and then call
> pcpu_alloc_noprof() with GFP_NOFS. It will try to acquire
> pcpu_alloc_mutex and block, creating the reverse dependency:
> 
> FS lock -> pcpu_alloc_mutex
> This can still form a potential deadlock cycle.

Correct.

> Does anyone have a good suggestion for how to handle this remaining case?
> Or should we simply treat all GFP_KERNEL/GFP_NOFS allocation behavior in
> pcpu_alloc_noprof() as GFP_NOIO?

Yes, weakening the reclaim context would work around these dependencies.
This seems like a viable option as long as pcp allocations are not in
latency sensitive paths and stalling them under memory pressure is
acceptable. My insight into pcp allocator users is very limited so I
cannot really make any judgment call here.

-- 
Michal Hocko
SUSE Labs