On Fri, Aug 11, 2017 at 06:45:49PM +0100, Ben Hutchings 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 <mgorman@xxxxxxx> > > > > 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. > In this context, the primary concern is a race with clearing and checking PTEs at the location protected by a single PTL lock. While the flag in question is a per-mm flag, the ordering only matters when a race can potentially occur. -- Mel Gorman SUSE Labs