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

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Date:   Sat, 18 Sep 2021 14:39:49 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Christoph Hellwig <hch@....de>
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>
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

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.