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Subject[tip: x86/mm] x86/mm: Ensure forced page table splitting
The following commit has been merged into the x86/mm branch of tip:

Commit-ID: 3e844d842d49cdbe61a4b338bdd512654179488a
Author: Dave Hansen <>
AuthorDate: Fri, 18 Nov 2022 07:16:16 -08:00
Committer: Dave Hansen <>
CommitterDate: Thu, 15 Dec 2022 10:37:28 -08:00

x86/mm: Ensure forced page table splitting

There are a few kernel users like kfence that require 4k pages to work
correctly and do not support large mappings. They use set_memory_4k()
to break down those large mappings.

That, in turn relies on cpa_data->force_split option to indicate to
set_memory code that it should split page tables regardless of whether
the need to be.

But, a recent change added an optimization which would return early
if a set_memory request came in that did not change permissions. It
did not consult ->force_split and would mistakenly optimize away the
splitting that set_memory_4k() needs. This broke kfence.

Skip the same-permission optimization when ->force_split is set.

Fixes: 127960a05548 ("x86/mm: Inhibit _PAGE_NX changes from cpa_process_alias()")
Signed-off-by: Dave Hansen <>
Tested-by: Marco Elver <>
Cc: Peter Zijlstra (Intel) <>
arch/x86/mm/pat/set_memory.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 220361c..0db6951 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -1727,7 +1727,8 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
* No changes, easy!
- if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)))
+ if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)) &&
+ !cpa->force_split)
return ret;

while (rempages) {
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