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

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