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

[Andrew Morton]

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.