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

[Kaitao Cheng]

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.

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?

If there is no clear solution for now, would it be acceptable to first
fix some of the issues introduced by commit 9a5b183941b, and leave this
remaining case as a pre-existing historical issue to be handled separately
later?

-- 
Thanks
Kaitao Cheng