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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Message-ID: <20211221085623.GA7733@lst.de>
Date:   Tue, 21 Dec 2021 09:56:23 +0100
From:   Christoph Hellwig <hch@....de>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, hch@....de,
        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 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.