Re: kmap_local semantics

From: Ira Weiny
Date: Wed Mar 17 2021 - 22:45:18 EST

On Wed, Mar 17, 2021 at 02:59:01PM +0100, Thomas Gleixner wrote:
> On Fri, Mar 12 2021 at 07:36, Ira Weiny wrote:
> > On Fri, Mar 12, 2021 at 07:54:13AM +0100, Christoph Hellwig wrote:
> >> So with the new kmap_local interface is it possible / advisable to
> >> use local kmaps over code that might schedule(), e.g. to wait for I/O?
> >
> > It is possible yes. "Advisable" I think so. Thomas knows better than I what
> > performance would be.
> The performance impact is only relevant for 32bit machines with HIGHMEM
> enabled. On 64bit kmap_local is a NOOP except when you enable
> CONFIG_DEBUG_KMAP_LOCAL there which then actually does the mapping (on
> x86 only for now) with guard maps between and that will do the 'drop
> map' on schedule out and restore on schedule in dance. But for regular
> 64bit there is absolutely _NO_ overhead.

Thanks for clarification.

> > FWIW I have been working on converting kmaps to kmap_local. Most of the
> > instances don't schedule AFAICT.
> The whole point of kmap_local is to be preemptible and does not have the
> nasty side effects of kmap_atomic, except for the 1 page per map and map
> nesting ordering requirements.
> > What I really don't want to see is any kmap'ings handed to another thread. I
> > am working hard to eliminate the use of kmap for that use. Is that going to be
> > a problem?
> kmap != kmap_local
> kmap_local mappings are thread local and can't be handed anywhere. It's
> documented.

Agreed. I'm Sorry, I did not word the above clearly enough. Let me rephrase

Christoph, do you anticipate additional need to call kmap and hand the mappings
to other threads? If not then kmap_local is what you should use. If so, I'd
like to know why.


> Thanks,
> tglx