Re: scatterwalk_map_and_copy incorrect optimization

From: Herbert Xu
Date: Sat Dec 10 2016 - 03:08:55 EST

On Fri, Dec 09, 2016 at 02:18:01PM +0100, Jason A. Donenfeld wrote:
> Hi Herbert,
> The scatterwalk_map_and_copy function copies ordinary buffers to and
> from scatterlists. These buffers can, of course, be on the stack, and
> this remains the most popular use of this function -- getting info
> between stack buffers and DMA regions. It's mostly used for adding or
> checking MACs, in the majority of call sites. Its implementation is
> relatively straightforward. It maps the DMA region(s) to a vaddr, and
> then just calls vanilla memcpy. Pretty uncontroversial.
> However, around ~4.1 an optimization was added to prevent copying when
> unnecessary (when the src and dst are the same). The optimization
> looks like this:
> if (sg_page(sg) == virt_to_page(buf) &&
> sg->offset == offset_in_page(buf))
> return;
> There are two problems with this:

This code no longer exists in the current tree.

Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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