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

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Date:   Wed, 25 Aug 2021 18:15:53 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Mike Rapoport <rppt@...nel.org>,
        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 Wed, Aug 25, 2021 at 01:12:37PM +0200, David Hildenbrand wrote:
> On 25.08.21 12:55, Catalin Marinas wrote:
> > On Wed, Aug 25, 2021 at 12:38:31PM +0200, David Hildenbrand wrote:
> > > On 25.08.21 12:20, Catalin Marinas wrote:
> > > > I can see the documentation for pfn_valid() does not claim anything more
> > > > than the presence of an memmap entry. But I wonder whether the confusion
> > > > is wider-spread than just the DMA code. At a quick grep, try_ram_remap()
> > > > assumes __va() can be used on pfn_valid(), though I suspect it relies on
> > > > the calling function to check that the resource was RAM. The arm64
> > > > kern_addr_valid() returns true based on pfn_valid() and kcore.c uses
> > > > standard memcpy on it, which wouldn't work for I/O (should we change
> > > > this check to pfn_is_map_memory() for arm64?).
> > > 
> > > kern_addr_valid() checks that there is a direct map entry, and that the
> > > mapped address has a valid mmap. (copied from x86-64)
> > 
> > It checks that there is a va->pa mapping, not necessarily in the linear
> > map as it walks the page tables. So for some I/O range that happens to
> > be mapped but which was in close proximity to RAM so that pfn_valid() is
> > true, kern_addr_valid() would return true. I don't thin that was the
> > intention.
> > 
> > > Would you expect to have a direct map for memory holes and similar (IOW,
> > > !System RAM)?
> > 
> > No, but we with the generic pfn_valid(), it may return true for mapped
> > MMIO (with different attributes than the direct map).
> 
> Ah, right. But can we actually run into that via kcore?
> 
> kcore builds the RAM list via walk_system_ram_range(), IOW the resource
> tree. And we end up calling kern_addr_valid() only on KCORE_RAM,
> KCORE_VMEMMAP and KCORE_TEXT.

It's probably fine but I'd rather do some check of the other call sites
before attempting to move arm64 to the generic pfn_valid() again.

-- 
Catalin