Re: FAILED: patch "[PATCH] mm: fix possible PMD dirty bit lost in" failed to apply to 4.14-stable tree

[Sasha Levin <sashal@kernel.org>]

On Mon, Mar 09, 2020 at 08:15:15PM +0100, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.14-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From 8a8683ad9ba48b4b52a57f013513d1635c1ca5c4 Mon Sep 17 00:00:00 2001
>From: Huang Ying <ying.huang@intel.com>
>Date: Thu, 5 Mar 2020 22:28:29 -0800
>Subject: [PATCH] mm: fix possible PMD dirty bit lost in
> set_pmd_migration_entry()
>
>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.

As d52605d7cb30 ("mm: do not lose dirty and accessed bits in
pmdp_invalidate()") has an explicit note asking not to backport it, we
won't backport it or this patch to 4.14.

-- 
Thanks,
Sasha