Re: [PATCH] dma/debug: Fix dma vs cow-page collision detection

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Nov 19, 2019 at 9:49 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> The debug_dma_assert_idle() infrastructure was put in place to catch a
> data corruption scenario first identified by the now defunct NET_DMA
> receive offload feature. It caught cases where dma was in flight to a
> stale page because the dma raced the cpu writing the page, and the cpu
> write triggered cow_user_page().
>
> However, the dma-debug tracking is overeager and also triggers in cases
> where the dma device is reading from a page that is also undergoing
> cow_user_page().
>
> The fix proposed was originally posted in 2016, and Russell reported
> "Yes, that seems to avoid the warning for me from an initial test", and
> now Don is also reporting that this fix is addressing a similar false
> positive report that he is seeing.
>
> Link: https://lore.kernel.org/r/CAPcyv4j8fWqwAaX5oCdg5atc+vmp57HoAGT6AfBFwaCiv0RbAQ@mail.gmail.com
> Reported-by: Russell King <linux@armlinux.org.uk>
> Reported-by: Don Dutile <ddutile@redhat.com>
> Fixes: 0abdd7a81b7e ("dma-debug: introduce debug_dma_assert_idle()")
> Cc: <stable@vger.kernel.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  kernel/dma/debug.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 099002d84f46..11a6db53d193 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -587,7 +587,7 @@ void debug_dma_assert_idle(struct page *page)
>         }
>         spin_unlock_irqrestore(&radix_lock, flags);
>
> -       if (!entry)
> +       if (!entry || entry->direction != DMA_FROM_DEVICE)
>                 return;
>
>         cln = to_cacheline_number(entry);

If I am understanding right DMA_TO_DEVICE is fine, but won't  you also
need to cover the DMA_BIDIRECTIONAL case since it is possible for a
device to also write the memory in that case?