Re: [PATCH 4.4 18/58] mm, mprotect: flush TLB if potentially racing with a parallel reclaim leaving stale TLB entries

[Mel Gorman]

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