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

[Thomas Gleixner]

On Tue, Sep 15 2020 at 17:05, Herbert Xu 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);
> }

Hmm, that still has the issue that the code between map and unmap must
not sleep and the conversion must carefully check whether anything in
this region relies on preemption being disabled by kmap_atomic()
regardless of highmem or not.

kmap_atomic() is at least consistent vs. preemption, the above not so
much.

I'd rather go for a preemptible/sleepable version of highmem mapping
which is in itself consistent for both highmen and not highmem.

Thanks,

        tglx