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

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>
This commit is contained in:
Michael Ellerman 2018-08-14 20:48:22 +10:00
parent 5c6499b704
commit 3b5657ed5b
1 changed files with 2 additions and 2 deletions
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4
arch/powerpc/mm/pgtable-radix.c
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@ -295,14 +295,14 @@ 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;
psize = mmu_virtual_psize;
}