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(-) > > > > > > > > >
