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Date:   Wed, 25 Aug 2021 11:55:11 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Mike Rapoport <rppt@kernel.org>,
        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: <20210825105510.GB3420@arm.com>
References: <d3a3c828-b777-faf8-e901-904995688437@gmail.com>
 <20210824173741.GC623@arm.com>
 <YSU6NVZ3j0XCurWC@kernel.org>
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 <60a11eba-2910-3b5f-ef96-97d4556c1596@redhat.com>
 <20210825102044.GA3420@arm.com>
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On Wed, Aug 25, 2021 at 12:38:31PM +0200, David Hildenbrand wrote:
> On 25.08.21 12:20, Catalin Marinas wrote:
> > On Tue, Aug 24, 2021 at 08:59:22PM +0200, David Hildenbrand wrote:
> > > On 24.08.21 20:46, Robin Murphy wrote:
> > > > On 2021-08-24 19:28, Mike Rapoport wrote:
> > > > > On Tue, Aug 24, 2021 at 06:37:41PM +0100, Catalin Marinas wrote:
> > > > > > On Tue, Aug 24, 2021 at 03:40:47PM +0200, Alex Bee wrote:
> > > > > > > it seems there is a regression in arm64 memory mapping in 5.14, since it
> > > > > > > fails on Rockchip RK3328 when the pl330 dmac tries to map with:
> > > > > > > 
> > > > > > >   ------------[ cut here ]------------
> > > > > > >   WARNING: CPU: 2 PID: 373 at kernel/dma/mapping.c:235 dma_map_resource+0x68/0xc0
> > > > > > >   Modules linked in: spi_rockchip(+) fuse
> > > > > > >   CPU: 2 PID: 373 Comm: systemd-udevd Not tainted 5.14.0-rc7 #1
> > > > > > >   Hardware name: Pine64 Rock64 (DT)
> > > > > > >   pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
> > > > > > >   pc : dma_map_resource+0x68/0xc0
> > > > > > >   lr : pl330_prep_slave_fifo+0x78/0xd0
> > > > > > >   sp : ffff800012102ae0
> > > > > > >   x29: ffff800012102ae0 x28: ffff000005c94800 x27: 0000000000000000
> > > > > > >   x26: ffff000000566bd0 x25: 0000000000000001 x24: 0000000000000001
> > > > > > >   x23: 0000000000000002 x22: ffff000000628c00 x21: 0000000000000001
> > > > > > >   x20: ffff000000566bd0 x19: 0000000000000001 x18: 0000000000000000
> > > > > > >   x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
> > > > > > >   x14: 0000000000000277 x13: 0000000000000001 x12: 0000000000000000
> > > > > > >   x11: 0000000000000001 x10: 00000000000008e0 x9 : ffff800012102a80
> > > > > > >   x8 : ffff000000d14b80 x7 : ffff0000fe7b12f0 x6 : ffff0000fe7b1100
> > > > > > >   x5 : fffffc000000000f x4 : 0000000000000000 x3 : 0000000000000001
> > > > > > >   x2 : 0000000000000001 x1 : 00000000ff190800 x0 : ffff000000628c00
> > > > > > >   Call trace:
> > > > > > >     dma_map_resource+0x68/0xc0
> > > > > > >     pl330_prep_slave_sg+0x58/0x220
> > > > > > >     rockchip_spi_prepare_dma+0xd8/0x2c0 [spi_rockchip]
> > > > > > >     rockchip_spi_transfer_one+0x294/0x3d8 [spi_rockchip]
> > > > > > [...]
> > > > > > > Note: This does not relate to the spi driver - when disabling this device in
> > > > > > > the device tree it fails for any other (i2s, for instance) which uses dma.
> > > > > > > Commenting out the failing check at [1], however, helps and the mapping
> > > > > > > works again.
> > > > > 
> > > > > > Do you know which address dma_map_resource() is trying to map (maybe
> > > > > > add some printk())? It's not supposed to map RAM, hence the warning.
> > > > > > Random guess, the address is 0xff190800 (based on the x1 above but the
> > > > > > regs might as well be mangled).
> > > > > 
> > > > > 0xff190800 will cause this warning for sure. It has a memory map, but it is
> > > > > not RAM so old version of pfn_valid() would return 0 and the new one
> > > > > returns 1.
> > > > 
> > > > How does that happen, though? It's not a memory address, and it's not
> > > > even within the bounds of anywhere there should or could be memory. This
> > > > SoC has a simple memory map - everything from 0 to 0xfeffffff goes to
> > > > the DRAM controller (which may not all be populated, and may have pieces
> > > > carved out by secure firmware), while 0xff000000-0xffffffff is MMIO. Why
> > > > do we have pages (or at least the assumption of pages) for somewhere
> > > > which by all rights should not have them?
> > > 
> > > Simple: we allocate the vmemmap for whole sections (e.g., 128 MiB) to avoid
> > > any such hacks. If there is a memory hole, it gets a memmap as well.
> > > 
> > > Tricking pfn_valid() into returning "false" where we actually have a memmap
> > > only makes it look like there is no memmap; but there is one, and
> > > it's PG_reserved.
> > 
> > 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).

> > > > > > Either pfn_valid() gets confused in 5.14 or something is wrong with the
> > > > > > DT. I have a suspicion it's the former since reverting the above commit
> > > > > > makes it disappear.
> > > > > 
> > > > > I think pfn_valid() actually behaves as expected but the caller is wrong
> > > > > because pfn_valid != RAM (this applies btw to !arm64 as well).
> > > > > 
> > > > > 	/* Don't allow RAM to be mapped */
> > > > > 	if (WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
> > > > > 		return DMA_MAPPING_ERROR;
> > > > > 
> > > > > Alex, can you please try this patch:
> > > > 
> > > > That will certainly paper over the issue, but it's avoiding the question
> > > > of what went wrong with the memory map in the first place. The comment
> > > > is indeed a bit inaccurate, but ultimately dma_map_resource() exists for
> > > > addresses that would be wrong to pass to dma_map_page(), so I believe
> > > > pfn_valid() is still the correct check.
> > > 
> > > If we want to check for RAM, pfn_valid() would be wrong. If we want to check
> > > for "is there a memmap, for whatever lives or does not live there",
> > > pfn_valid() is the right check.
> > 
> > So what should the DMA code use instead? Last time we needed something
> > similar, the recommendation was to use pfn_to_online_page(). Mike is
> > suggesting memblock_is_memory().
> 
> We use pfn_to_online_page() when we want to know if it's system RAM and that
> the memmap actually contains something sane (-> memmap content has a well
> defined state).
> 
> You can have offline memory blocks where pfn_to_online_page() would return
> "false", memblock_is_memory() would return "true". IOW, there is a memmap,
> it's System RAM, but the memmap is stale and not trustworthy.
> 
> If you want to make sure no System RAM (online/offline/...) will get mapped,
> memblock_is_memory() should be the right thing to use. I recall that x86
> traverse the resource tree instead to exclude system ram regions similarly.

Thanks, this makes sense.

-- 
Catalin