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

[Zhao Liu]

On Mon, Oct 03, 2022 at 01:24:51PM -0700, Ira Weiny wrote:
> Date: Mon, 3 Oct 2022 13:24:51 -0700
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page()
> 
> On Mon, Oct 03, 2022 at 07:43:49PM +0200, Fabio M. De Francesco wrote:
> > On Wednesday, September 28, 2022 11:56:40 AM CEST 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.
> > > 
> > > In addtion, the fuction hyperv_init() checks if kmap() fails by BUG_ON().
> > > From the original discussion[2], the BUG_ON() here is just used to
> > > explicitly panic NULL pointer. So still keep the BUG_ON() in place to check
> > > if kmap_local_page() fails. 
> > 
> > How might kmap_local_page() return invalid kernel addresses? 
> > 
> > I think that, if this function returns, the pointer is always a valid kernel 
> > address. Am I missing something?
> 
> I think this is my mistake.  I did not realize what Zhao was asking me the
> other day and generally said to leave BUG_ON's alone and not change things.

Thanks Fabio and Ira! This is my negligence, my consideration is still not
thorough enough. I'll remove this BUG_ON().

> 
> In this case kmap_local_page() will not return NULL.  It will BUG on its own if
> it fails.
> 
> > 
> > > Based on this consideration, memcpy_to_page()
> > > is not selected here but only kmap_local_page() is used.
> > 
> > I can't agree with you, if the premises are that kmap_local_page() might 
> > provide invalid addresses.
>  
> I'm ok with the patch as is.  But Fabio is correct and it may be worth removing
> the check in a follow on patch.
> Ira
> 
> > Thanks,
> > 
> > Fabio
> > 
> > > Therefore, replace kmap() with kmap_local_page() in hyperv/hv_init.c.
> > > 
> > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx
> > > [2]: https://lore.kernel.org/lkml/20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2/
> > > 
> > > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> > > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > Suggested-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > > Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
> > > 
> > > ---
> > > Suggested by credits.
> > > 	Dave: Referred to his comments about whether kmap() can fail and 
> > the
> > > 	      suggestion to keep BUG_ON() in place.
> > > 	Ira: Referred to his task documentation and review comments about
> > > 	     keeping BUG_ON() for kmap_local_page().
> > > 	Fabio: Stole some of his boiler plate commit message.
> > > ---
> > >  arch/x86/hyperv/hv_init.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > 
> > 
> >