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

[Kaitao Cheng <kaitao.cheng@linux.dev>]

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.

Kaitao Cheng (2):
  mm/percpu: Preserve NOFS/NOIO scope during chunk create and populate
  mm/percpu: Avoid pcpu_alloc_mutex recursion from reclaim

 mm/percpu.c | 39 +++++++++++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 4 deletions(-)

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2.50.1 (Apple Git-155)