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? > 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. 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(-) >
