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Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)
To:     Christoph Hellwig <hch@lst.de>, Mike Rapoport <rppt@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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        linux-mm@kvack.org,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <71b7d1b5-4a5f-9053-b22b-4d946cbf6d6e@redhat.com>
Date:   Mon, 20 Sep 2021 13:13:32 +0200
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On 18.09.21 07:18, Christoph Hellwig wrote:
> On Sat, Sep 18, 2021 at 12:22:47AM +0300, Mike Rapoport wrote:
>> I did some digging and it seems that the most "generic" way to check if a
>> page is in RAM is page_is_ram(). It's not 100% bullet proof as it'll give
>> false negatives for architectures that do not register "System RAM", but
>> those are not using dma_map_resource() anyway and, apparently, never would.
> 
> The downside of page_is_ram is that it looks really expensiv for
> something done at dma mapping time.
> 

There would be ways to speed it up, similar to

https://lkml.kernel.org/r/20210902160919.25683-2-david@redhat.com

but the end result is still walking a list. Question would be, how much 
that overhead matters in practice.

-- 
Thanks,

David / dhildenb