linux-kernel - Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool

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Message-ID: <40c60196-b30c-359f-e1a2-171a6be45296@linux.alibaba.com>
Date:   Wed, 17 Aug 2022 10:03:02 +0800
From:   Liu Song <liusong@...ux.alibaba.com>
To:     Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        akpm@...ux-foundation.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Christoph Hellwig <hch@....de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within
 dma_pool_alloc

> On 2022-08-16 13:39, Marek Szyprowski wrote:
>> Hi,
>>
>> On 18.07.2022 08:28, Liu Song wrote:
>>> From: Liu Song <liusong@...ux.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@...ux.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