Ben Hutchings <ben.hutchi...@codethink.co.uk> wrote: > On Wed, 2017-08-09 at 12:41 -0700, Greg Kroah-Hartman wrote: >> 4.4-stable review patch. If anyone has any objections, please let me know. >> >> ------------------ >> >> From: Mel Gorman <mgor...@suse.de> >> >> commit 3ea277194daaeaa84ce75180ec7c7a2075027a68 upstream. > [...] >> +/* >> + * Reclaim unmaps pages under the PTL but do not flush the TLB prior to >> + * releasing the PTL if TLB flushes are batched. It's possible for a >> parallel >> + * operation such as mprotect or munmap to race between reclaim unmapping >> + * the page and flushing the page. If this race occurs, it potentially >> allows >> + * access to data via a stale TLB entry. Tracking all mm's that have TLB >> + * batching in flight would be expensive during reclaim so instead track >> + * whether TLB batching occurred in the past and if so then do a flush here >> + * if required. This will cost one additional flush per reclaim cycle paid >> + * by the first operation at risk such as mprotect and mumap. >> + * >> + * This must be called under the PTL so that an access to tlb_flush_batched >> + * that is potentially a "reclaim vs mprotect/munmap/etc" race will >> synchronise >> + * via the PTL. > > What about USE_SPLIT_PTE_PTLOCKS? I don't see how you can use "the PTL" > to synchronise access to a per-mm flag.
Although it is a per-mm flag, the only situations we care about it are those in which “the PTL” (i.e. the same PTL) is accessed by both the reclaimer (which batches the flushes) and mprotect/munmap/etc. Nadav
