powerpc/mm/radix: Fix overuse of small pages in splitting logic

BugLink: https://bugs.launchpad.net/bugs/1854975
[ 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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index 5aedfdad582737d6f1ce2a0844cd592a1b2e3138..10fbd98c0f8a8799c19f50def2462c0f6b686b50 100644 (file)
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/arch/powerpc/mm/pgtable-radix.c
@@ -244,14 +244,14 @@ retry:
 
                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;
 
                if (mapping_size != previous_size) {