Messages in this thread | | | Date | Wed, 17 Aug 2022 10:03:02 +0800 | Subject | Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool_alloc | From | Liu Song <> |
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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.
Hi,
First of all, I am very sorry that there are missing branches that have not been considered fully,
but there is a possibility that the memset to 0 will be repeated whether re-allocation from the
dma pool, and this patch needs to be fixed.
Thanks
> > 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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