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.
