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

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Message-Id: <B1F6A370-1AA6-43CE-8284-42B326846EA5@gmail.com>
Date:   Sat, 12 Aug 2017 23:27:23 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     Mel Gorman <mgorman@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 4.4 18/58] mm, mprotect: flush TLB if potentially racing
 with a parallel reclaim leaving stale TLB entries

Ben Hutchings <ben.hutchings@...ethink.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 <mgorman@...e.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