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

[Ben Hutchings]

On Sat, 2017-08-12 at 23:27 -0700, Nadav Amit wrote:
> Ben Hutchings <ben.hutchings@xxxxxxxxxxxxxxx> 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.
> 
> Although it is a per-mm flag, the only situations we care about it are those
> in which âthe PTLâ (i.e. the same PTL) is accessed by both the reclaimer
> (which batches the flushes) and mprotect/munmap/etc.

Is there anything that presents this sequence?

P0                              P1                             P2
--                              --                             --

change_pte_range() [ptl=X]
-> flush_tlb_batch_pending()
   -> flush_tlb_mm()
                                try_to_unmap_one() [ptl=Y]
                                -> set_tlb_ubc_flush_pending()
                                   -> tlb_flush_batched = true
   -> tlb_flush_batched = false

change_pte_range() [ptl=Y]
                                                               ->
flush_tlb_batch_pending()
                                                                  (nop)

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.