Re: copy_page() on a kmalloc-ed page with DEBUG_SLAB enabled (was "zram: do not use copy_page with non-page alinged address")

[Matthew Wilcox]

On Wed, Apr 19, 2017 at 03:02:37PM +0900, Minchan Kim wrote:
> On Tue, Apr 18, 2017 at 09:33:07AM +0200, Michal Hocko wrote:
> > I do not follow. Why would you need kmap for something that is already
> > in the kernel space?
> 
> Because it can work with highmem pages.

That's copy_user_highpage().  If you want to define a new arch API
copy_highpage(), feel free to make a case for it ...

> > > Another approach is the API does normal thing for non-aligned prefix and
> > > tail space and fast thing for aligned space.
> > > Otherwise, it would be happy if the API has WARN_ON non-page SIZE aligned
> > > address.

Why not just use memcpy()?  Is copy_page() significantly faster than
memcpy() for a PAGE_SIZE amount of data?