Messages in this thread Patch in this message |  | | Date | Mon, 18 Jul 2022 00:42:24 +0800 | From | "liusong" <> | Subject | Re: [PATCH] mm/dmapool.c: avoid duplicate memset withi n dma pool alloc |
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>> From: Liu Song <liusong@linux.alibaba.com> >> >> In "dma_direct_alloc", the allocated memory is explicitly set to 0. >> If use direct alloc, we need to avoid possible duplicate memset in >> dma_pool_alloc. > >I'm having trouble seeing how this change is safe and correct and >maintainable. Please describe the code flow more completely? The following is the code flow, dma_pool_alloc |--> pool_alloc_page |--> dma_alloc_coherent |--> dma_alloc_attrs In "dma_alloc_attrs", if "dma_alloc_direct" is true, then enter "dma_direct_alloc", and in "dma_direct_alloc", as long as the memory allocation is successful, will execute "memset(ret, 0, size);", which set memory to zero. Kernel use "dma_go_direct" to determine whether to use direct allocation, which mainly by judging whether "dma_map_ops" exists.
So this patch determines whether direct alloc will be used by judging does "dma_map_ops" exists, thereby avoiding repeated memset.
> >> --- 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 +373,7 @@ 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) && get_dma_ops(pool->dev)) >> memset(retval, 0, pool->size); > >That DMAPOOL_DEBUG memset a couple of lines earlier could/should be >testing the same condition - there's no point in poisoning an area >which we're about to zero out.
If DMAPOOL_DEBUG is configured, its logic is internally self-consistent. If the user needs __GFP_ZERO, the corresponding memory will be set to 0. |  |