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Date:   Thu, 18 Aug 2022 16:37:44 +0800
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Subject: Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within
 dma_pool_alloc
To:     Christoph Hellwig <hch@lst.de>,
        Robin Murphy <robin.murphy@arm.com>, akpm@linux-foundation.org
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>
References: <1658125690-76930-1-git-send-email-liusong@linux.alibaba.com>
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 <413d8666-7a82-efd7-6716-13658016ca10@arm.com>
 <20220817053628.GA28747@lst.de>
From:   Liu Song <liusong@linux.alibaba.com>
In-Reply-To: <20220817053628.GA28747@lst.de>
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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".
> WTF?  get_dma_ops is privat to the DMA API layer, and dmapool has no
> business even using that.  Even independent of this particular case,
> consumers of an API never have any business looking at the implementation
> of the API, that is the whole point of the abstraction.
>
>> 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?
> And yes, in addition to that it also is completely broken.

After reading everyone's comments, I found that fixing this patch will

make the code look strange, so the benefits of the changes will be

dispensable, so I also agree to discard this patch.

Sorry for this trouble again.


Thanks