Re: Regression after commit 19809c2da28a ("mm, vmalloc: use __GFP_HIGHMEM implicitly")

[Laura Abbott]

On 02/08/2018 05:06 AM, Matthew Wilcox wrote:
> On Thu, Feb 08, 2018 at 02:29:57PM +0800, Kai Heng Feng wrote:
> > A user with i386 instead of AMD64 machine reports [1] that commit 19809c2da28a ("mm, vmalloc: use __GFP_HIGHMEM implicitlyâ) causes a regression.
> > BUG_ON(PageHighMem(pg)) in drivers/media/common/saa7146/saa7146_core.c always gets triggered after that commit.
>
> Well, the BUG_ON is wrong.  You can absolutely have pages which are both
> HighMem and under the 4GB boundary.  Only the first 896MB (iirc) are LowMem,
> and the next 3GB of pages are available to vmalloc_32().
>
> > Also there are other BUG_ON(PageHighMem()) in drivers/media, I think they will get hit by same regression in 32bit machine too.
>
> I fixed one of them.  I think the other three are also bogus, but it's
> hard to say; the comments say "DMA to HighMem might not work", and they
> probably mean "above the 4GB boundary", but I really don't know.
>
> (since two drivers now have this code, maybe it should be part of the core
> MM API?  Or maybe there's already something better they should be using?)

The comment at the top said the function was copied from videobuf-dma-sg.c
so might be worth making it common.

> diff --git a/drivers/media/common/saa7146/saa7146_core.c b/drivers/media/common/saa7146/saa7146_core.c
> index 9f7c5b0a6b45..329fd43228ff 100644
> --- a/drivers/media/common/saa7146/saa7146_core.c
> +++ b/drivers/media/common/saa7146/saa7146_core.c
> @@ -160,7 +160,7 @@ static struct scatterlist* vmalloc_to_sg(unsigned char *virt, int nr_pages)
>   		pg = vmalloc_to_page(virt);
>   		if (NULL == pg)
>   			goto err;
> -		BUG_ON(PageHighMem(pg));
> +		BUG_ON(page_to_pfn(pg) >= (1 << (32 - PAGE_SHIFT)));
>   		sg_set_page(&sglist[i], pg, PAGE_SIZE, 0);
>   	}
>   	return sglist;
> diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
> index f412429cf5ba..b5ec74b9c867 100644
> --- a/drivers/media/v4l2-core/videobuf-dma-sg.c
> +++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
> @@ -77,7 +77,7 @@ static struct scatterlist *videobuf_vmalloc_to_sg(unsigned char *virt,
>   		pg = vmalloc_to_page(virt);
>   		if (NULL == pg)
>   			goto err;
> -		BUG_ON(PageHighMem(pg));
> +		BUG_ON(page_to_pfn(pg) >= (1 << (32 - PAGE_SHIFT)));
>   		sg_set_page(&sglist[i], pg, PAGE_SIZE, 0);
>   	}
>   	return sglist;

the vzalloc in this function needs to be switched to vmalloc32 if it
actually wants to guarantee 32-bit memory.

Thanks,
Laura