Re: [PATCH 0/2] mm/percpu: Fix possible NOFS/NOIO reclaim recursion

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 May 2026 21:29:15 +0800 Kaitao Cheng <kaitao.cheng@linux.dev> wrote:

> Commit 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations
> atomic") allowed GFP_NOFS and GFP_NOIO percpu allocations to use
> pcpu_alloc_mutex and the chunk creation slow path. This restored the
> allocation capability that was lost when those constrained allocations
> were treated as atomic, but it also opens two possible reclaim recursion
> problems.
> 
> The first problem is that the create and populate slow paths do not fully
> preserve the caller's allocation constraints. pcpu_alloc_noprof() derives
> pcpu_gfp from the caller supplied GFP mask and passes it to the backing
> page allocator. However, pcpu_create_chunk() calls pcpu_get_vm_areas(),
> and population can allocate temporary metadata or page tables while mapping
> backing pages. Those internal allocations can use GFP_KERNEL. A caller
> using GFP_NOFS or GFP_NOIO can therefore still enter unconstrained FS or
> IO reclaim while holding pcpu_alloc_mutex. This defeats the caller's
> allocation context.
> 
> The second problem is a possible pcpu_alloc_mutex recursion from reclaim.
> If reclaim is entered while pcpu_alloc_mutex is already held, and reclaim
> reaches a path which allocates percpu memory with GFP_NOFS or GFP_NOIO,
> the nested allocation can now try to take pcpu_alloc_mutex again because
> 9a5b183941b5 no longer treats those masks as atomic.
> 
> Another possible way to avoid these issues is to revert 9a5b183941b5.
> However, that would also bring back the premature allocation failures for
> sleepable GFP_NOFS/GFP_NOIO percpu users that 9a5b183941b5 was intended
> to fix.

Thanks.

9a5b183941b5 has been in there for a year.  How are you observing/triggering this bug
and what are the userspace-visible effects?

We might choose to backport fixes into -stable kernels, but this additional info
is needed to make that determination.