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    SubjectRe: [PATCH v1 0/2] nvme-pci: Fix dma-iommu mapping failures when PAGE_SIZE=64KB
    On Wed, Feb 14, 2024 at 11:57:32AM -0800, Nicolin Chen wrote:
    > On Wed, Feb 14, 2024 at 04:41:38PM +0000, Will Deacon wrote:
    > > On Tue, Feb 13, 2024 at 01:53:55PM -0800, Nicolin Chen wrote:
    > > > It's observed that an NVME device is causing timeouts when Ubuntu boots
    > > > with a kernel configured with PAGE_SIZE=64KB due to failures in swiotlb:
    > > > systemd[1]: Started Journal Service.
    > > > => nvme 0000:00:01.0: swiotlb buffer is full (sz: 327680 bytes), total 32768 (slots), used 32 (slots)
    > > > note: journal-offline[392] exited with irqs disabled
    > > > note: journal-offline[392] exited with preempt_count 1
    > > >
    > > > An NVME device under a PCIe bus can be behind an IOMMU, so dma mappings
    > > > going through dma-iommu might be also redirected to swiotlb allocations.
    > > > Similar to dma_direct_max_mapping_size(), dma-iommu should implement its
    > > > dma_map_ops->max_mapping_size to return swiotlb_max_mapping_size() too.
    > > >
    > > > Though an iommu_dma_max_mapping_size() is a must, it alone can't fix the
    > > > issue. The swiotlb_max_mapping_size() returns 252KB, calculated from the
    > > > default pool 256KB subtracted by min_align_mask NVME_CTRL_PAGE_SIZE=4KB,
    > > > while dma-iommu can roundup a 252KB mapping to 256KB at its "alloc_size"
    > > > when PAGE_SIZE=64KB via iova->granule that is often set to PAGE_SIZE. So
    > > > this mismatch between NVME_CTRL_PAGE_SIZE=4KB and PAGE_SIZE=64KB results
    > > > in a similar failure, though its signature has a fixed size "256KB":
    > > > systemd[1]: Started Journal Service.
    > > > => nvme 0000:00:01.0: swiotlb buffer is full (sz: 262144 bytes), total 32768 (slots), used 128 (slots)
    > > > note: journal-offline[392] exited with irqs disabled
    > > > note: journal-offline[392] exited with preempt_count 1
    > > >
    > > > Both failures above occur to NVME behind IOMMU when PAGE_SIZE=64KB. They
    > > > were likely introduced for the security feature by:
    > > > commit 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers"),
    > > >
    > > > So, this series bundles two fixes together against that. They should be
    > > > taken at the same time to entirely fix the mapping failures.
    > >
    > > It's a bit of a shot in the dark, but I've got a pending fix to some of
    > > the alignment handling in swiotlb. It would be interesting to know if
    > > patch 1 has any impact at all on your NVME allocations:
    > >
    > > https://lore.kernel.org/r/20240205190127.20685-1-will@kernel.org
    >
    > I applied these three patches locally for a test.

    Thank you!

    > Though I am building with a v6.6 kernel, I see some warnings:
    > from kernel/dma/swiotlb.c:26:
    > kernel/dma/swiotlb.c: In function ‘swiotlb_area_find_slots’:
    > ./include/linux/minmax.h:21:35: warning: comparison of distinct pointer types lacks a cast
    > 21 | (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
    > | ^~
    > ./include/linux/minmax.h:27:18: note: in expansion of macro ‘__typecheck’
    > 27 | (__typecheck(x, y) && __no_side_effects(x, y))
    > | ^~~~~~~~~~~
    > ./include/linux/minmax.h:37:31: note: in expansion of macro ‘__safe_cmp’
    > 37 | __builtin_choose_expr(__safe_cmp(x, y), \
    > | ^~~~~~~~~~
    > ./include/linux/minmax.h:75:25: note: in expansion of macro ‘__careful_cmp’
    > 75 | #define max(x, y) __careful_cmp(x, y, >)
    > | ^~~~~~~~~~~~~
    > kernel/dma/swiotlb.c:1007:26: note: in expansion of macro ‘max’
    > 1007 | stride = max(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
    > | ^~~
    >
    > Replacing with a max_t() can fix these.

    Weird, I haven't seen that. I can fix it as you suggest, but please can
    you also share your .config so I can look into it further?

    > And it seems to get worse, as even a 64KB mapping is failing:
    > [ 0.239821] nvme 0000:00:01.0: swiotlb buffer is full (sz: 65536 bytes), total 32768 (slots), used 0 (slots)
    >
    > With a printk, I found the iotlb_align_mask isn't correct:
    > swiotlb_area_find_slots:alloc_align_mask 0xffff, iotlb_align_mask 0x800
    >
    > But fixing the iotlb_align_mask to 0x7ff still fails the 64KB
    > mapping..

    Hmm. A mask of 0x7ff doesn't make a lot of sense given that the slabs
    are 2KiB aligned. I'll try plugging in some of the constants you have
    here, as something definitely isn't right...

    Will

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