Messages in this thread | | | Date | Tue, 16 Aug 2022 16:00:27 +0100 | Subject | Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool_alloc | From | Robin Murphy <> |
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On 2022-08-16 13:39, Marek Szyprowski wrote: > Hi, > > On 18.07.2022 08:28, Liu Song wrote: >> From: Liu Song <liusong@linux.alibaba.com> >> >> In "dma_alloc_from_dev_coherent" and "dma_direct_alloc", >> the allocated memory is explicitly set to 0. >> >> A helper function "use_dev_coherent_memory" is introduced here to >> determine whether the memory is allocated by "dma_alloc_from_dev_coherent". >> >> And use "get_dma_ops" to determine whether the memory is allocated by >> "dma_direct_alloc". >> >> After this modification, memory allocated using "dma_pool_zalloc" can avoid >> duplicate memset. >> >> Signed-off-by: Liu Song <liusong@linux.alibaba.com> > > This patch landed linux next-20220816. Unfortunately it causes serious > issues on ARM 32bit systems. I've observed it on ARM 32bit Samsung > Exynos 5422 based Odroid XU4 board with USB r8152 driver. After applying > this patch and loading r8152 driver I only the following endless > messages in the log: > > xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued? > xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued? > xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued? > > It looks that there are drivers which rely on the fact that the dma > coherent buffers are always zeroed.
It's not even that, the change here is just obviously broken, since it ends up entirely ignoring want_init_on_alloc() for devices using dma-direct. Sure, the memory backing a dma_page is zeroed *once* by its initial dma-coherent allocation, but who says we're not not reallocating pool entries from an existing dma_page?
I'm not convinced it's worth trying to special-case this at all, since we can only do it reliably for the first pool entry allocated from a new dma_page, and that will only happen as the pool initially grows to a suitable size for its working set, after which no further new pages are likely to be allocated for the lifetime of the pool. Even if there is a case to be made for doing so, it would need to be based on the flow through dma_pool_alloc() itself, not some nonsense heuristic on the device.
Andrew, please drop this patch.
Thanks, Robin.
>> --- >> include/linux/dma-map-ops.h | 5 +++++ >> mm/dmapool.c | 5 ++++- >> 2 files changed, 9 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h >> index 0d5b06b..c29948d 100644 >> --- a/include/linux/dma-map-ops.h >> +++ b/include/linux/dma-map-ops.h >> @@ -171,6 +171,10 @@ int dma_alloc_from_dev_coherent(struct device *dev, ssize_t size, >> int dma_release_from_dev_coherent(struct device *dev, int order, void *vaddr); >> int dma_mmap_from_dev_coherent(struct device *dev, struct vm_area_struct *vma, >> void *cpu_addr, size_t size, int *ret); >> +static inline bool use_dev_coherent_memory(struct device *dev) >> +{ >> + return dev->dma_mem ? true : false; >> +} >> #else >> static inline int dma_declare_coherent_memory(struct device *dev, >> phys_addr_t phys_addr, dma_addr_t device_addr, size_t size) >> @@ -180,6 +184,7 @@ static inline int dma_declare_coherent_memory(struct device *dev, >> #define dma_alloc_from_dev_coherent(dev, size, handle, ret) (0) >> #define dma_release_from_dev_coherent(dev, order, vaddr) (0) >> #define dma_mmap_from_dev_coherent(dev, vma, vaddr, order, ret) (0) >> +#define use_dev_coherent_memory(dev) (0) >> #endif /* CONFIG_DMA_DECLARE_COHERENT */ >> >> #ifdef CONFIG_DMA_GLOBAL_POOL >> diff --git a/mm/dmapool.c b/mm/dmapool.c >> index a7eb5d0..6e03530 100644 >> --- a/mm/dmapool.c >> +++ b/mm/dmapool.c >> @@ -21,6 +21,7 @@ >> >> #include <linux/device.h> >> #include <linux/dma-mapping.h> >> +#include <linux/dma-map-ops.h> >> #include <linux/dmapool.h> >> #include <linux/kernel.h> >> #include <linux/list.h> >> @@ -372,7 +373,9 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags, >> #endif >> spin_unlock_irqrestore(&pool->lock, flags); >> >> - if (want_init_on_alloc(mem_flags)) >> + if (want_init_on_alloc(mem_flags) && >> + !use_dev_coherent_memory(pool->dev) && >> + get_dma_ops(pool->dev)) >> memset(retval, 0, pool->size); >> >> return retval; > > Best regards
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