[PATCH AUTOSEL 4.19 093/237] powerpc/mm/radix: Fix overuse of small pages in splitting logic

Sasha Levin
2019-11-16T16:22:57Z

From: Michael Ellerman <mpe@ellerman.id.au>

[ Upstream commit 3b5657ed5b4e27ccf593a41ff3c5aa27dae8df18 ]

When we have CONFIG_STRICT_KERNEL_RWX enabled, we want to split the
linear mapping at the text/data boundary so we can map the kernel text
read only.

But the current logic uses small pages for the entire text section,
regardless of whether a larger page size would fit. eg. with the
boundary at 16M we could use 2M pages, but instead we use 64K pages up
to the 16M boundary:

Mapped 0x0000000000000000-0x0000000001000000 with 64.0 KiB pages
Mapped 0x0000000001000000-0x0000000040000000 with 2.00 MiB pages
Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages

This is because the test is checking if addr is < __init_begin
and addr + mapping_size is >= _stext. But that is true for all pages
between _stext and __init_begin.

Instead what we want to check is if we are crossing the text/data
boundary, which is at __init_begin. With that fixed we see:

Mapped 0x0000000000000000-0x0000000000e00000 with 2.00 MiB pages
Mapped 0x0000000000e00000-0x0000000001000000 with 64.0 KiB pages
Mapped 0x0000000001000000-0x0000000040000000 with 2.00 MiB pages
Mapped 0x0000000040000000-0x0000000100000000 with 1.00 GiB pages

ie. we're correctly using 2MB pages below __init_begin, but we still
drop down to 64K pages unnecessarily at the boundary.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/mm/pgtable-radix.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index 24a2eadc8c21a..b387c7b917b7e 100644
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/arch/powerpc/mm/pgtable-radix.c
@@ -295,14 +295,14 @@ static int __meminit create_physical_mapping(unsigned long start,

if (split_text_mapping && (mapping_size == PUD_SIZE) &&
(addr < __pa_symbol(__init_begin)) &&
- (addr + mapping_size) >= __pa_symbol(_stext)) {
+ (addr + mapping_size) >= __pa_symbol(__init_begin)) {
max_mapping_size = PMD_SIZE;
goto retry;
}

if (split_text_mapping && (mapping_size == PMD_SIZE) &&
(addr < __pa_symbol(__init_begin)) &&
- (addr + mapping_size) >= __pa_symbol(_stext)) {
+ (addr + mapping_size) >= __pa_symbol(__init_begin)) {
mapping_size = PAGE_SIZE;
psize = mmu_virtual_psize;
}
--
2.20.1