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

Peter Zijlstra
2017-08-11T10:50:25Z

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.