On Mon, Mar 27, 2023 at 12:08:20PM +0200, Fabio M. De Francesco wrote: > 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(). > > > > 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. Or should we just stop allocating aio rings from HIGHMEM and remove the calls to kmap()? How much memory are we talking about here?
