linux-kernel - Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

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Message-ID: <20210918051843.GA16104@lst.de>
Date:   Sat, 18 Sep 2021 07:18:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        David Hildenbrand <david@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        Alex Bee <knaerzche@...il.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

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.