[PATCH v6 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables
From: Nicolas Boichat
Date: Sun Dec 09 2018 - 20:15:47 EST
This is a follow-up to the discussion in , .
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. 
This series is the most memory-efficient approach.
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?).
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
- 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 .
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(-)