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

From: Mike Rapoport <rppt@kernel.org>
To: Christoph Hellwig <hch@lst.de>
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>
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)
Date: Sat, 18 Sep 2021 14:39:49 +0300	[thread overview]
Message-ID: <YUXQBV6KKi+/Q6gX@kernel.org> (raw)
In-Reply-To: <YUWlO2tZC5IwCAHV@kernel.org>

On Sat, Sep 18, 2021 at 11:37:22AM +0300, Mike Rapoport wrote:
> On Sat, Sep 18, 2021 at 07:18:43AM +0200, 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.
> 
> Indeed :(
> But pfn_valid is plain wrong...
> I'll keep digging.

I did some more archaeology and it that check for pfn_valid() was requested
by arm folks because their MMU may have troubles with alias mappings with
different attributes and so they made the check to use a false assumption
that pfn_valid() == "RAM".

As this WARN_ON(pfn_valid()) is only present in dma_map_resource() it's
probably safe to drop it entirely. 

Otherwise the simplest way would be to hide it behind something like
ARCH_WANTS_DMA_NOT_RAM and make arm/arm64 select it.

Thoughts?

-- 
Sincerely yours,
Mike.