Re: [PATCH v2 2/2] ACPI: sysfs: Expose BDAT data region

[Jamie Nguyen <jamien@nvidia.com>]

The Sashiko review of this patch [1] notes that acpi_data_show() maps
the whole data region on every read(), and that the unmaps are deferred
by RCU, so repeated reads could pile up mappings and exhaust vmalloc
space.

That code isn't changed by this series. acpi_data_show() has mapped the
whole region on every read since BERT support was added in 7dae6326ed76
("ACPI / sysfs: Extend ACPI sysfs to provide access to boot error
region"), and BERT and CCEL use the same path.

I measured it with v2 on an arm64 server, with 4K and 64K pages and the
90754-byte BDAT region:

- Looping full-file reads, with 1 to 64 readers, kept at most about 4500
  deferred mappings (426 MiB) in flight. The count stayed flat and went
  back to baseline within 2 s of stopping.
- The worst case, tight loops of 1-byte reads, peaked at about 34k
  mappings and 4.6 GiB of address space, and also drained within 2 s.

At those rates, exhausting the 126 TiB of vmalloc space would need the
deferred unmaps to stop completing for more than half an hour.

If it's worth tightening anyway, acpi_data_show() could map only the
range being read, which caps each mapping at two pages for all three
tables. I can send that as a separate patch.

[1] https://sashiko.dev/#/patchset/20261012230632.3574987-1-jamien%40nvidia.com

Jamie