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

[Ben Hutchings]

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.

Ben.

> + */
> +void flush_tlb_batched_pending(struct mm_struct *mm)
> +{
> +	if (mm->tlb_flush_batched) {
> +		flush_tlb_mm(mm);
> +
> +		/*
> +		 * Do not allow the compiler to re-order the clearing of
> +		 * tlb_flush_batched before the tlb is flushed.
> +		 */
> +		barrier();
> +		mm->tlb_flush_batched = false;
> +	}
> +}
>  #else
>  static void set_tlb_ubc_flush_pending(struct mm_struct *mm,
>  		struct page *page, bool writable)
> 
> 

-- 
Ben Hutchings
Software Developer, Codethink Ltd.