Re: [PATCH] mm/percpu: allow embed allocator when total size fits in vmalloc

[Paul Sherman <shermanpauldylan@gmail.com>]

Andrew,

Thank you for the encouraging words on the initial submission.

Tejun,

Further testing on SG2042 (Sv39, 4 NUMA nodes, 128GB) showed that
the proposed change allows the embedded first chunk to be created
successfully, but later dynamic chunk population consistently fails
in pcpu_populate_chunk(), with warnings originating from
__vmap_pages_range_noflush(). Reverting the patch eliminates those
warnings on the same hardware.

Looking through the code, my current understanding is that dynamic
chunks inherit the first chunk's layout through the shared
pcpu_unit_offsets[] table, while vmalloc reservations are driven by
pcpu_group_offsets[]. That led me to wonder whether I've encountered
a fundamental allocator invariant rather than merely an implementation
detail.

Is the requirement that all chunks share a single pcpu_unit_offsets[]
layout fundamental to the percpu allocator design, or is there an
intended mechanism for decoupling the embedded first-chunk layout from
dynamic chunk vmalloc mappings?

The answer will help determine whether the correct next step is to
withdraw the patch and instead document the existing invariant, or
whether there's a larger design direction worth investigating.

Thanks,
Paul