Re: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with kmap_local_page()

Jeff Moyer
2022-10-19T15:43:29Z

"Fabio M. De Francesco" <fmdefrancesco@gmail.com> writes:

> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
>
> Since its use in fs/aio.c is safe everywhere, it should be preferred.

That sentence is very ambiguous. I don't know what "its" refers to, and
I'm not sure what "safe" means in this context.

The patch looks okay to me.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>