On giovedì 19 gennaio 2023 17:20:55 CEST Fabio M. De Francesco wrote: > The use of kmap() and kmap_atomic() are being deprecated in favor of > kmap_local_page(). According to a suggestion by Matthew, I just sent another patch which stops allocating aio rings from ZONE_HIGHMEM.[1] Therefore, please drop this patch. Since the purpose of the new patch is entirely different from this, I changed the subject and reset the version number to v1. Thanks, Fabio [1] https://lore.kernel.org/lkml/20230609145937.17610-1-fmdefrancesco@xxxxxxxxx/ > There are two main problems with kmap(): (1) It comes with an overhead as > the 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 still valid. > > The use of kmap_local_page() in fs/aio.c is "safe" in the sense that the > code don't hands the returned kernel virtual addresses to other threads > and there are no nesting which should be handled with the stack based > (LIFO) mappings/un-mappings order. Furthermore, the code between the old > kmap_atomic()/kunmap_atomic() did not depend on disabling page-faults > and/or preemption, so that there is no need to call pagefault_disable() > and/or preempt_disable() before the mappings. > > Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in > fs/aio.c. > > Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel > with HIGHMEM64GB enabled. > > Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@xxxxxxxxx> > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx> > Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx> > Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx> > Reviewed-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> > ---
