Re: [PATCH v6 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables

[Nicolas Boichat <drinkcat@chromium.org>]

Hi all,

On Mon, Dec 10, 2018 at 9:15 AM Nicolas Boichat <drinkcat@chromium.org> wrote:
>
> This is a follow-up to the discussion in [1], [2].
>
> IOMMUs using ARMv7 short-descriptor format require page tables
> (level 1 and 2) to be allocated within the first 4GB of RAM, even
> on 64-bit systems.
>
> For L1 tables that are bigger than a page, we can just use __get_free_pages
> with GFP_DMA32 (on arm64 systems only, arm would still use GFP_DMA).
>
> For L2 tables that only take 1KB, it would be a waste to allocate a full
> page, so we considered 3 approaches:
>  1. This series, adding support for GFP_DMA32 slab caches.
>  2. genalloc, which requires pre-allocating the maximum number of L2 page
>     tables (4096, so 4MB of memory).
>  3. page_frag, which is not very memory-efficient as it is unable to reuse
>     freed fragments until the whole page is freed. [3]
>
> This series is the most memory-efficient approach.

Does anyone have any further comment on this series? If not, which
maintainer is going to pick this up? I assume Andrew Morton?

Thanks,

> stable@ note:
>   We confirmed that this is a regression, and IOMMU errors happen on 4.19
>   and linux-next/master on MT8173 (elm, Acer Chromebook R13). The issue
>   most likely starts from commit ad67f5a6545f ("arm64: replace ZONE_DMA
>   with ZONE_DMA32"), i.e. 4.15, and presumably breaks a number of Mediatek
>   platforms (and maybe others?).
>
> [1] https://lists.linuxfoundation.org/pipermail/iommu/2018-November/030876.html
> [2] https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031696.html
> [3] https://patchwork.codeaurora.org/patch/671639/
>
> Changes since v1:
>  - Add support for SLAB_CACHE_DMA32 in slab and slub (patches 1/2)
>  - iommu/io-pgtable-arm-v7s (patch 3):
>    - Changed approach to use SLAB_CACHE_DMA32 added by the previous
>      commit.
>    - Use DMA or DMA32 depending on the architecture (DMA for arm,
>      DMA32 for arm64).
>
> Changes since v2:
>  - Reworded and expanded commit messages
>  - Added cache_dma32 documentation in PATCH 2/3.
>
> v3 used the page_frag approach, see [3].
>
> Changes since v4:
>  - Dropped change that removed GFP_DMA32 from GFP_SLAB_BUG_MASK:
>    instead we can just call kmem_cache_*alloc without GFP_DMA32
>    parameter. This also means that we can drop PATCH v4 1/3, as we
>    do not make any changes in GFP flag verification.
>  - Dropped hunks that added cache_dma32 sysfs file, and moved
>    the hunks to PATCH v5 3/3, so that maintainer can decide whether
>    to pick the change independently.
>
> Changes since v5:
>  - Rename ARM_V7S_TABLE_SLAB_CACHE to ARM_V7S_TABLE_SLAB_FLAGS.
>  - Add stable@ to cc.
>
> Nicolas Boichat (3):
>   mm: Add support for kmem caches in DMA32 zone
>   iommu/io-pgtable-arm-v7s: Request DMA32 memory, and improve debugging
>   mm: Add /sys/kernel/slab/cache/cache_dma32
>
>  Documentation/ABI/testing/sysfs-kernel-slab |  9 +++++++++
>  drivers/iommu/io-pgtable-arm-v7s.c          | 19 +++++++++++++++----
>  include/linux/slab.h                        |  2 ++
>  mm/slab.c                                   |  2 ++
>  mm/slab.h                                   |  3 ++-
>  mm/slab_common.c                            |  2 +-
>  mm/slub.c                                   | 16 ++++++++++++++++
>  tools/vm/slabinfo.c                         |  7 ++++++-
>  8 files changed, 53 insertions(+), 7 deletions(-)
>
> --
> 2.20.0.rc2.403.gdbc3b29805-goog
>