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. > Thanks, > Zhao
