Re: [PATCH] mm/sparse: Fix race on mem_section->usage in pfn walkers

[Muchun Song <muchun.song@linux.dev>]

> On Apr 15, 2026, at 16:04, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> On 4/15/26 04:23, Muchun Song wrote:
>> When memory is hot-removed, section_deactivate() can tear down
>> mem_section->usage while concurrent pfn walkers still inspect the
>> subsection map via pfn_section_valid() or pfn_section_first_valid().
> 
> I'll note that it's all racy either way: someone checking pfn_valid() /
> pfn_to_online_page() can race with concurrent unplug.

Agree. When I first saw the commit message for 5ec8e8ea8b77, I was curious
because the goal of this commit was to fix an access issue with ms->usage.
Looking at the race diagram, I realized that while this only addresses the
->usage access, subsequent accesses to struct page will still be problematic.
It's just that the former issue happened to be triggered first in this specific
commit.

> 
> We've known that for years; it's hard to fix; it never ever triggers :)

Glad to know my analysis wasn't off! It seems I've just stumbled upon a
'well-known secret' within the community. :)

> 
> So is this really worth it, when we should in fact, work on protecting
> the users of pfn_valid() / pfn_to_online_page() with rcu or similar?

I am not sure if it is worth fixing, especially since I just realized the
community has been aware of this issue for many years. If we do decide to
fix it, I think the most straightforward approach would be to protect it
using RCU, something like:

# the user side of pfn_to_online_page():
	rcu_read_lock(); 
	page = pfn_to_online_page();
	if (!get_page_unless_zero(page))
	    goto out_unlock;
	rcu_read_unlock();


# the vmemmap freeing side should free the vmemmap pages via RCU.


Thanks,
Muchun

> 
> -- 
> Cheers,
> 
> David