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Date:   Fri, 27 Sep 2019 14:59:45 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Qian Cai <cai@lca.pw>
Cc:     heiko.carstens@de.ibm.com, gor@linux.ibm.com,
        borntraeger@de.ibm.com, linux-s390@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: fix a crash in free_pages_prepare()
Message-Id: <20190927145945.846a3f3405d3af066827d3f5@linux-foundation.org>
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On Fri, 27 Sep 2019 17:28:06 -0400 Qian Cai <cai@lca.pw> wrote:

> >=20
> > So I think you've moved the arch_free_page() to be after the final
> > thing which can access page contents, yes?  If so, we should have a
> > comment in free_pages_prepare() to attmept to prevent this problem from
> > reoccurring as the code evolves?
>=20
> Right, something like this above arch_free_page() there?
>=20
> /*
>  * It needs to be just above=A0kernel_map_pages(), as s390 could mark tho=
se
>  * pages unused and then trigger a fault when accessing.
>  */

I did this.

--- a/mm/page_alloc.c~mm-page_alloc-fix-a-crash-in-free_pages_prepare-fix
+++ a/mm/page_alloc.c
@@ -1179,7 +1179,13 @@ static __always_inline bool free_pages_p
 		kernel_init_free_pages(page, 1 << order);
=20
 	kernel_poison_pages(page, 1 << order, 0);
+	/*
+	 * arch_free_page() can make the page's contents inaccessible.  s390
+	 * does this.  So nothing which can access the page's contents should
+	 * happen after this.
+	 */
 	arch_free_page(page, order);
+
 	if (debug_pagealloc_enabled())
 		kernel_map_pages(page, 1 << order, 0);
=20
_