Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally

[Ard Biesheuvel]

On Tue, 15 Sep 2020 at 10:05, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 14, 2020 at 11:55:53PM -0700, Linus Torvalds wrote:
> >
> > Maybe we could hide it behind a debug option, at least.
> >
> > Or, alterantively, introduce a new "debug_preempt_count" that doesn't
> > actually disable preemption, but warns about actual sleeping
> > operations..
>
> I'm more worried about existing users of kmap_atomic relying on
> the preemption disabling semantics.  Short of someone checking
> on every single instance (and that would include derived cases
> such as all users of sg miter), I think the safer option is to
> create something brand new and then migrate the existing users
> to it.  Something like
>
> static inline void *kmap_atomic_ifhigh(struct page *page)
> {
>         if (PageHighMem(page))
>                 return kmap_atomic(page);
>         return page_address(page);
> }
>
> static inline void kunmap_atomic_ifhigh(struct page *page, void *addr)
> {
>         if (PageHighMem(page))
>                 kunmap_atomic(addr);
> }
>

But we would still need to check all users of SG miter before we could
move it to this interface.