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Date:   Sat, 18 Sep 2021 07:18:43 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Alex Bee <knaerzche@gmail.com>, Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm@kvack.org,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)
Message-ID: <20210918051843.GA16104@lst.de>
References: <d3a3c828-b777-faf8-e901-904995688437@gmail.com> <20210824173741.GC623@arm.com> <YSU6NVZ3j0XCurWC@kernel.org> <0908ce39-7e30-91fa-68ef-11620f9596ae@arm.com> <60a11eba-2910-3b5f-ef96-97d4556c1596@redhat.com> <20210825102044.GA3420@arm.com> <YUUHJ5EI+6Mop4Wu@kernel.org>
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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.