Date:   Tue, 14 Dec 2021 11:28:01 +0100 (CET)
From:   Christoph Lameter <cl@gentwo.org>
To:     Vlastimil Babka <vbabka@suse.cz>
cc:     Baoquan He <bhe@redhat.com>, Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, hch@lst.de, 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>
Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed
 pages in DMA zone
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References: <20211213122712.23805-1-bhe@redhat.com> <20211213122712.23805-6-bhe@redhat.com> <20211213134319.GA997240@odroid> <20211214053253.GB2216@MiWiFi-R3L-srv> <f5ff82eb-73b6-55b5-53d7-04ab73ce5035@suse.cz>
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On Tue, 14 Dec 2021, Vlastimil Babka wrote:

> If doesn't feel right to me to fix (or rather workaround) this on the level
> of kmalloc caches just because the current reports come from there. If we
> decide it's acceptable for kdump kernel to return !ZONE_DMA memory for
> GFP_DMA requests, then it should apply at the page allocator level for all
> allocations, not just kmalloc().
>
> Also you mention above you'd prefer ZONE_DMA32 memory, while chances are
> this approach of using KMALLOC_NORMAL caches will end up giving you
> ZONE_NORMAL. On the page allocator level it would be much easier to
> implement a fallback from non-populated ZONE_DMA to ZONE_DMA32 specifically.

Well this only works if the restrictions on the physical memory addresses
of each platform make that possible.