On Wed, Nov 04, 2020 at 07:02:20PM +0100, Vlastimil Babka wrote:

On 11/3/20 5:20 PM, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

Subject should have "on DEBUG_PAGEALLOC" ?

quoted

The design of DEBUG_PAGEALLOC presumes that __kernel_map_pages() must never
fail. With this assumption is wouldn't be safe to allow general usage of
this function.

Moreover, some architectures that implement __kernel_map_pages() have this
function guarded by #ifdef DEBUG_PAGEALLOC and some refuse to map/unmap
pages when page allocation debugging is disabled at runtime.

As all the users of __kernel_map_pages() were converted to use
debug_pagealloc_map_pages() it is safe to make it available only when
DEBUG_PAGEALLOC is set.

Signed-off-by: Mike Rapoport <redacted>
Acked-by: David Hildenbrand <redacted>
Acked-by: Kirill A. Shutemov <redacted>
---
  arch/Kconfig                     |  3 +++
  arch/arm64/Kconfig               |  4 +---
  arch/arm64/mm/pageattr.c         |  8 ++++++--
  arch/powerpc/Kconfig             |  5 +----
  arch/riscv/Kconfig               |  4 +---
  arch/riscv/include/asm/pgtable.h |  2 --
  arch/riscv/mm/pageattr.c         |  2 ++
  arch/s390/Kconfig                |  4 +---
  arch/sparc/Kconfig               |  4 +---
  arch/x86/Kconfig                 |  4 +---
  arch/x86/mm/pat/set_memory.c     |  2 ++
  include/linux/mm.h               | 10 +++++++---
  12 files changed, 26 insertions(+), 26 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 56b6ccc0e32d..56d4752b6db6 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -1028,6 +1028,9 @@ config HAVE_STATIC_CALL_INLINE
  	bool
  	depends on HAVE_STATIC_CALL
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	bool
+
  source "kernel/gcov/Kconfig"
  source "scripts/gcc-plugins/Kconfig"

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1d466addb078..a932810cfd90 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -71,6 +71,7 @@ config ARM64
  	select ARCH_USE_QUEUED_RWLOCKS
  	select ARCH_USE_QUEUED_SPINLOCKS
  	select ARCH_USE_SYM_ANNOTATIONS
+	select ARCH_SUPPORTS_DEBUG_PAGEALLOC
  	select ARCH_SUPPORTS_MEMORY_FAILURE
  	select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
  	select ARCH_SUPPORTS_ATOMIC_RMW

@@ -1025,9 +1026,6 @@ config HOLES_IN_ZONE
  source "kernel/Kconfig.hz"
-config ARCH_SUPPORTS_DEBUG_PAGEALLOC
-	def_bool y
-
  config ARCH_SPARSEMEM_ENABLE
  	def_bool y
  	select SPARSEMEM_VMEMMAP_ENABLE

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 1b94f5b82654..439325532be1 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c

@@ -155,7 +155,7 @@ int set_direct_map_invalid_noflush(struct page *page)
  		.clear_mask = __pgprot(PTE_VALID),
  	};
-	if (!rodata_full)
+	if (!debug_pagealloc_enabled() && !rodata_full)
  		return 0;
  	return apply_to_page_range(&init_mm,

@@ -170,7 +170,7 @@ int set_direct_map_default_noflush(struct page *page)
  		.clear_mask = __pgprot(PTE_RDONLY),
  	};
-	if (!rodata_full)
+	if (!debug_pagealloc_enabled() && !rodata_full)
  		return 0;
  	return apply_to_page_range(&init_mm,

I don't understand these two hunks. Previous patch calls this for
hibernation when CONFIG_ARCH_HAS_SET_DIRECT_MAP, which is true for arm64.
Why suddenly this starts to depend on debug_pagealloc_enabled()?

I was confused about this for quite a long :)

On arm64 the changes to direct^w linear map are allowed when 

	debug_page_alloc() || rodata_full

In hibernation we essentially have now

	if (1)
		set_direct_map(something)
	else
		debug_page_alloc_map()

With debug_pagealloc enabled but with rodata_full disabled arm64
versions of set_direct_map_*() will become a nop, so a page that was
unmapped by debug_pagealloc() will not be mapped back.

I'm still puzzled how hibernation might ever need to save a free page,
but that's another story.

-- 
Sincerely yours,
Mike.

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