linux-kernel - Re: [PATCH v6 3/7] Revert "mm: numa: defer TLB flush for THP migration as long as possible"

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Message-ID: <20170811105015.4njdpy3il76g5uuk@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Aug 2017 12:50:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     linux-mm@...ck.org, nadav.amit@...il.com,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        Minchan Kim <minchan@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v6 3/7] Revert "mm: numa: defer TLB flush for THP
 migration as long as possible"

On Tue, Aug 01, 2017 at 05:08:14PM -0700, Nadav Amit wrote:
> While deferring TLB flushes is a good practice, the reverted patch
> caused pending TLB flushes to be checked while the page-table lock is
> not taken. As a result, in architectures with weak memory model (PPC),
> Linux may miss a memory-barrier, miss the fact TLB flushes are pending,
> and cause (in theory) a memory corruption.
> 
> Since the alternative of using smp_mb__after_unlock_lock() was
> considered a bit open-coded, and the performance impact is expected to
> be small, the previous patch is reverted.

FWIW this Changelog sucks arse; you completely fail to explain the
broken ordering.