linux-kernel - Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone

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Date:   Thu, 23 Dec 2021 09:52:38 +0100
From:   Christoph Hellwig <hch@....de>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Christoph Hellwig <hch@....de>, Baoquan He <bhe@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, cl@...ux.com,
        John.p.donnelly@...cle.com, kexec@...ts.infradead.org,
        stable@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no
 managed pages in DMA zone

On Wed, Dec 22, 2021 at 12:37:03PM +0000, Hyeonggon Yoo wrote:
> Oh I misunderstood this. Underlying physical address of vmalloc()-allocated memory
> can be mapped using DMA API, and it needs to be setup as scatterlist because
> the allocated memory is not physically continuous. Right?

Yes.

> BTW, looking at the API I think the scsi case can be converted to use
> dma_alloc_pages(). but driver requires 512 bytes of buffer and the API
> supports allocating by at least page size.

Overallocating is not generally a problem, but if the allocations are for
a slow path it might make more sense to stick to dma_map_* and bounce
buffer if needed.

> It's not a big problem as it allocates a single buffer but in other
> cases maybe not. Can't we use dma pool for non-coherent pages?

No.