Hi Nicolin,

On Thu, May 23, 2019 at 09:06:32PM -0700, Nicolin Chen wrote:
> Both dma_alloc_from_contiguous() and dma_release_from_contiguous()
> are very simply implemented, but requiring callers to pass certain
> parameters like count and align, and taking a boolean parameter to
> check __GFP_NOWARN in the allocation flags. So every function call
> duplicates similar work:
>   /* A piece of example */
>   unsigned long order = get_order(size);
>   size_t count = size >> PAGE_SHIFT;
>   page = dma_alloc_from_contiguous(dev, count, order, gfp & __GFP_NOWARN);
>   [...]
>   dma_release_from_contiguous(dev, page, size >> PAGE_SHIFT);
> 
> Additionally, as CMA can be used only in the context which permits
> sleeping, most of callers do a gfpflags_allow_blocking() check and
> a corresponding fallback allocation of normal pages upon any false
> result:
>   /* A piece of example */
>   if (gfpflags_allow_blocking(flag))
>       page = dma_alloc_from_contiguous();
>   if (!page)
>       page = alloc_pages();
>   [...]
>   if (!dma_release_from_contiguous(dev, page, count))
>       __free_pages(page, get_order(size));
> 
> So this patch simplifies those function calls by abstracting these
> operations into the two new functions: dma_{alloc,free}_contiguous.
> 
> As some callers of dma_{alloc,release}_from_contiguous() might be
> complicated, this patch just implements these two new functions to
> kernel/dma/direct.c only as an initial step.
> 
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
> ---

This commit is causing boot failures in QEMU on x86_64 defconfig:

https://travis-ci.com/ClangBuiltLinux/continuous-integration/jobs/203825363

Attached is a bisect log and a boot log with GCC (just to show it is not
a compiler thing).

My QEMU command line is:

qemu-system-x86_64 -m 512m \
                   -drive file=images/x86_64/rootfs.ext4,format=raw,if=ide \
                   -append 'console=ttyS0 root=/dev/sda' \
                   -nographic \
                   -kernel arch/x86_64/boot/bzImage

and the rootfs is available here:

https://github.com/ClangBuiltLinux/continuous-integration/raw/master/images/x86_64/rootfs.ext4

I haven't seen a report on this yet so apologize if there is already a
fix in the works. Let me know if you need anythnig from me.

Cheers,
Nathan