Date:   Tue, 21 Dec 2021 09:56:23 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, hch@lst.de,
        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: <20211221085623.GA7733@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>
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On Fri, Dec 17, 2021 at 11:38:27AM +0000, Hyeonggon Yoo wrote:
> My understanding is any buffer requested from kmalloc (without
> GFP_DMA/DMA32) can be used by device driver because it allocates
> continuous physical memory. It doesn't mean that buffer allocated
> with kmalloc is free of addressing limitation.

Yes.

> 
> the addressing limitation comes from the capability of device, not
> allocation size. if you allocate memory using alloc_pages() or kmalloc(),
> the device has same limitation. and vmalloc can't be used for
> devices because they have no MMU.

vmalloc can be used as well, it just needs to be setup as a scatterlist
and needs a little lover for DMA challenged platforms with the
invalidate_kernel_vmap_range and flush_kernel_vmap_range helpers.

> But we can map memory outside DMA zone into bounce buffer (which resides
> in DMA zone) using DMA API.

Yes, although in a few specific cases the bounce buffer could also come
from somewhere else.