Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool_alloc

[Robin Murphy <robin.murphy@arm.com>]

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