Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page()

[Zhao Liu]

On Sun, Oct 09, 2022 at 08:46:33PM +0000, Wei Liu wrote:
> Date: Sun, 9 Oct 2022 20:46:33 +0000
> From: Wei Liu <wei.liu@xxxxxxxxxx>
> Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page()
> 
> On Thu, Oct 06, 2022 at 10:21:43PM +0800, Zhao Liu wrote:
> > On Mon, Oct 03, 2022 at 08:50:16AM +0000, Wei Liu wrote:
> > > Date: Mon, 3 Oct 2022 08:50:16 +0000
> > > From: Wei Liu <wei.liu@xxxxxxxxxx>
> > > Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page()
> > > 
> > > On Wed, Sep 28, 2022 at 02:08:40PM +0000, Wei Liu wrote:
> > > > On Wed, Sep 28, 2022 at 05:56:40PM +0800, Zhao Liu wrote:
> > > > > From: Zhao Liu <zhao1.liu@xxxxxxxxx>
> > > > > 
> > > > > kmap() is being deprecated in favor of kmap_local_page()[1].
> > > > > 
> > > > > There are two main problems with kmap(): (1) It comes with an overhead as
> > > > > 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 are still valid.
> > > > > 
> > > > > In the fuction hyperv_init() of hyperv/hv_init.c, the mapping is used in a
> > > > > single thread and is short live. So, in this case, it's safe to simply use
> > > > > kmap_local_page() to create mapping, and this avoids the wasted cost of
> > > > > kmap() for global synchronization.
> > > > > 
> > > > 
> > > > The kmap call in that function is not performance critical in any way,
> > > > and at this point in the initialization process I don't expect there to
> > > > be any contention, so the downside of kmap is not really a concern here.
> > > > 
> > > > That being said, kmap getting deprecated is a good enough reason to
> > > > switch to kmap_local_page. And I appreciate this well-written,
> > > > well-reasoned commit message.
> > > > 
> > > > I will apply it to hyperv-next later -- I doubt people will object to
> > > > this change, but just in case.
> > > 
> > > Applied to hyperv-next. Thanks.
> > 
> > Sorry Wei, based on Ira and Fabio's comments, do you agree me to send a
> > follow on patch to remove that BUG_ON()? Or send the v2 patch?
> > 
> 
> I just sent a PR to Linus. That PR includes your v1 patch.
> 
> While the code can be improved, BUG_ON that does nothing is harmless.
> You can send a follow-up patch to remove the BUG_ON.

Thanks Wei! I'll do that.

Regards,
Zhao

> 
> Thanks,
> Wei.
> 
> > Thanks,
> > Zhao