Re: [PATCH] mm: Fix possible PMD dirty bit lost in set_pmd_migration_entry()

From: Andrew Morton
Date: Thu Feb 20 2020 - 19:55:42 EST

On Thu, 20 Feb 2020 15:52:20 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:

> From: Huang Ying <ying.huang@xxxxxxxxx>
> In set_pmd_migration_entry(), pmdp_invalidate() is used to change PMD
> atomically. But the PMD is read before that with an ordinary memory
> reading. If the THP (transparent huge page) is written between the
> PMD reading and pmdp_invalidate(), the PMD dirty bit may be lost, and
> cause data corruption. The race window is quite small, but still
> possible in theory, so need to be fixed.
> The race is fixed via using the return value of pmdp_invalidate() to
> get the original content of PMD, which is a read/modify/write atomic
> operation. So no THP writing can occur in between.
> The race has been introduced when the THP migration support is added
> in the commit 616b8371539a ("mm: thp: enable thp migration in generic
> path"). But this fix depends on the commit d52605d7cb30 ("mm: do not
> lose dirty and accessed bits in pmdp_invalidate()"). So it's easy to
> be backported after v4.16. But the race window is really small, so it
> may be fine not to backport the fix at all.

Thanks. I'm inclined to add a cc:stable to this one. Silent data corruption is
pretty serious.