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Message-ID: <1506474463.7101.5.camel@mtkswgap22>
Date: Wed, 27 Sep 2017 09:07:43 +0800
From: Miles Chen <miles.chen@...iatek.com>
To: Robin Murphy <robin.murphy@....com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
<wsd_upstream@...iatek.com>, <linux-mediatek@...ts.infradead.org>,
<iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH] dma-debug: fix incorrect pfn calculation
On Tue, 2017-09-26 at 15:53 +0100, Robin Murphy wrote:
> On 26/09/17 14:24, miles.chen@...iatek.com wrote:
> > From: Miles Chen <miles.chen@...iatek.com>
> >
> > dma-debug report the following warning:
> >
> > [name:panic&]WARNING: CPU: 3 PID: 298 at kernel-4.4/lib/dma-debug.c:604
> > debug _dma_assert_idle+0x1a8/0x230()
> > DMA-API: cpu touching an active dma mapped cacheline [cln=0x00000882300]
> > CPU: 3 PID: 298 Comm: vold Tainted: G W O 4.4.22+ #1
> > Hardware name: MT6739 (DT)
> > Call trace:
> > [<ffffff800808acd0>] dump_backtrace+0x0/0x1d4
> > [<ffffff800808affc>] show_stack+0x14/0x1c
> > [<ffffff800838019c>] dump_stack+0xa8/0xe0
> > [<ffffff80080a0594>] warn_slowpath_common+0xf4/0x11c
> > [<ffffff80080a061c>] warn_slowpath_fmt+0x60/0x80
> > [<ffffff80083afe24>] debug_dma_assert_idle+0x1a8/0x230
> > [<ffffff80081dca9c>] wp_page_copy.isra.96+0x118/0x520
> > [<ffffff80081de114>] do_wp_page+0x4fc/0x534
> > [<ffffff80081e0a14>] handle_mm_fault+0xd4c/0x1310
> > [<ffffff8008098798>] do_page_fault+0x1c8/0x394
> > [<ffffff800808231c>] do_mem_abort+0x50/0xec
> >
> > I found that debug_dma_alloc_coherent() and debug_dma_free_coherent()
> > always use type "dma_debug_coherent" and assume that dma_alloc_coherent()
> > always returns a linear address.
> >
> > However if a device returns false on is_device_dma_coherent(),
> > dma_alloc_coherent() will create another non-cacheable mapping
> > (also non linear). In this case, page_to_pfn(virt_to_page(virt)) will
> > return an incorrect pfn. If the pfn is valid and mapped as a COW page,
> > we will hit the warning when doing wp_page_copy().
> >
> > Fix this by calculating correct pfn if is_device_dma_coherent()
> > returns false.
>
> As the inevitable storm of kbuild robot reports will tell you soon, you
> can't do that: is_device_dma_coherent() is a private helper between
> arm{,64} arch code and xen, and should not be used anywhere else.
thanks for that. I should not use the private helper function here.
> > Signed-off-by: Miles Chen <miles.chen@...iatek.com>
> > ---
> > lib/dma-debug.c | 20 ++++++++++++++------
> > 1 file changed, 14 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/dma-debug.c b/lib/dma-debug.c
> > index ea4cc3d..b17e56e 100644
> > --- a/lib/dma-debug.c
> > +++ b/lib/dma-debug.c
> > @@ -47,6 +47,8 @@ enum {
> > dma_debug_sg,
> > dma_debug_coherent,
> > dma_debug_resource,
> > + dma_debug_noncoherent,
> > + nr_dma_debug_types,
> > };
> >
> > enum map_err_types {
> > @@ -154,9 +156,9 @@ static inline bool dma_debug_disabled(void)
> > [MAP_ERR_CHECKED] = "dma map error checked",
> > };
> >
> > -static const char *type2name[5] = { "single", "page",
> > +static const char *type2name[nr_dma_debug_types] = { "single", "page",
> > "scather-gather", "coherent",
> > - "resource" };
> > + "resource", "noncoherent" };
> >
> > static const char *dir2name[4] = { "DMA_BIDIRECTIONAL", "DMA_TO_DEVICE",
> > "DMA_FROM_DEVICE", "DMA_NONE" };
> > @@ -1484,6 +1486,7 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
> > dma_addr_t dma_addr, void *virt)
> > {
> > struct dma_debug_entry *entry;
> > + bool coherent = is_device_dma_coherent(dev);
> >
> > if (unlikely(dma_debug_disabled()))
> > return;
> > @@ -1495,9 +1498,11 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
> > if (!entry)
> > return;
> >
> > - entry->type = dma_debug_coherent;
> > + entry->type = coherent ? dma_debug_coherent :
> > + dma_debug_noncoherent;
> > entry->dev = dev;
> > - entry->pfn = page_to_pfn(virt_to_page(virt));
>
> There are more architectures where the virtual address returned by
> dma_alloc_coherent is not a linear map address - some just have a static
> offset between cacheable and non-cacheable aliases - so there may be
> other cases where this is the wrong calculation, but at least those
> probably don't trigger the problematic false-positive.
>
> That said, the cases where coherent allocations *are* dynamically
> remapped should be easy enough to handle properly without having to
> resort to dodgy hacks:
>
> if (is_vmalloc_addr(virt))
> pfn = vmalloc_to_pfn(virt);
> else
> pfn = page_to_pfn(virt_to_page(virt));
>
> Simple.
Thanks for your advice. I'll try this approach.
Miles
>
> > + entry->pfn = coherent ? page_to_pfn(virt_to_page(virt)) :
> > + dma_addr >> PAGE_SHIFT;
>
> And in particular, this is just as likely to just give *different* false
> positives, since there's no guarantee whatsoever that dma_addr has any
> relationship to the appropriate pfn.
>
> Robin.
>
> > entry->offset = offset_in_page(virt);
> > entry->size = size;
> > entry->dev_addr = dma_addr;
> > @@ -1510,10 +1515,13 @@ void debug_dma_alloc_coherent(struct device *dev, size_t size,
> > void debug_dma_free_coherent(struct device *dev, size_t size,
> > void *virt, dma_addr_t addr)
> > {
> > + bool coherent = is_device_dma_coherent(dev);
> > struct dma_debug_entry ref = {
> > - .type = dma_debug_coherent,
> > + .type = coherent ? dma_debug_coherent :
> > + dma_debug_noncoherent,
> > .dev = dev,
> > - .pfn = page_to_pfn(virt_to_page(virt)),
> > + .pfn = coherent ? page_to_pfn(virt_to_page(virt)) :
> > + addr >> PAGE_SHIFT,
> > .offset = offset_in_page(virt),
> > .dev_addr = addr,
> > .size = size,
> >
>
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