Date:   Thu, 23 Dec 2021 09:52:38 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Christoph Hellwig <hch@lst.de>, Baoquan He <bhe@redhat.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, cl@linux.com,
        John.p.donnelly@oracle.com, kexec@lists.infradead.org,
        stable@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no
 managed pages in DMA zone
Message-ID: <20211223085238.GA7555@lst.de>
References: <20211213122712.23805-1-bhe@redhat.com> <20211213122712.23805-6-bhe@redhat.com> <20211213134319.GA997240@odroid> <20211214053253.GB2216@MiWiFi-R3L-srv> <Ybx2szXEgl1tN4MD@ip-172-31-30-232.ap-northeast-1.compute.internal> <20211221085623.GA7733@lst.de> <YcMb7+RESj1nEx9l@ip-172-31-30-232.ap-northeast-1.compute.internal>
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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.