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

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Date:   Thu, 18 Aug 2022 16:37:44 +0800
From:   Liu Song <liusong@...ux.alibaba.com>
To:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>, akpm@...ux-foundation.org
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, 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

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