[RFC V2] mm: Enable generic pfn_valid() to handle early sections with memmap holes

[Anshuman Khandual <anshuman.khandual@arm.com>]

Platforms like arm and arm64 have redefined pfn_valid() because their early
memory sections might have contained memmap holes after freeing parts of it
during boot, which should be skipped while validating a pfn for struct page
backing. This scenario on certain platforms where memmap is not continuous,
could be captured with a new option CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES.
Then the generic pfn_valid() can be improved to accommodate such platforms.
This reduces overall code footprint and also improves maintainability.

free_unused_memmap() and pfn_to_online_page() have been updated to include
such cases. This also exports memblock_is_memory() for all drivers that use
pfn_valid() but lack required visibility. After the new config is in place,
drop CONFIG_HAVE_ARCH_PFN_VALID from arm64 platforms.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This patch applies on the latest mainline kernel after Mike's series
regarding arm64 based pfn_valid().

https://lore.kernel.org/linux-mm/20210422061902.21614-1-rppt@kernel.org/T/#t

Changes in RFC V2:

- Dropped support for arm (32 bit)
- Replaced memblock_is_map_memory() check with memblock_is_memory()
- MEMBLOCK_NOMAP memory are no longer skipped for pfn_valid()
- Updated pfn_to_online_page() per David
- Updated free_unused_memmap() to preserve existing semantics per Mike
- Exported memblock_is_memory() instead of memblock_is_map_memory()

Changes in RFC V1:

- https://patchwork.kernel.org/project/linux-mm/patch/1615174073-10520-1-git-send-email-anshuman.khandual@arm.com/

 arch/arm64/Kconfig            |  2 +-
 arch/arm64/include/asm/page.h |  1 -
 arch/arm64/mm/init.c          | 41 -----------------------------------
 include/linux/mmzone.h        | 18 ++++++++++++++-
 mm/Kconfig                    |  9 ++++++++
 mm/memblock.c                 |  8 +++++--
 mm/memory_hotplug.c           |  5 +++++
 7 files changed, 38 insertions(+), 46 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b4a9b493ce72..4cdc3570ffa9 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -144,7 +144,6 @@ config ARM64
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
-	select HAVE_ARCH_PFN_VALID
 	select HAVE_ARCH_PREL32_RELOCATIONS
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_STACKLEAK
@@ -167,6 +166,7 @@ config ARM64
 		if $(cc-option,-fpatchable-function-entry=2)
 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
 		if DYNAMIC_FTRACE_WITH_REGS
+	select HAVE_EARLY_SECTION_MEMMAP_HOLES
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
index 75ddfe671393..fcbef3eec4b2 100644
--- a/arch/arm64/include/asm/page.h
+++ b/arch/arm64/include/asm/page.h
@@ -37,7 +37,6 @@ void copy_highpage(struct page *to, struct page *from);
 
 typedef struct page *pgtable_t;
 
-int pfn_valid(unsigned long pfn);
 int pfn_is_map_memory(unsigned long pfn);
 
 #include <asm/memory.h>
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index f431b38d0837..5731a11550d8 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -217,47 +217,6 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
 	free_area_init(max_zone_pfns);
 }
 
-int pfn_valid(unsigned long pfn)
-{
-	phys_addr_t addr = PFN_PHYS(pfn);
-
-	/*
-	 * Ensure the upper PAGE_SHIFT bits are clear in the
-	 * pfn. Else it might lead to false positives when
-	 * some of the upper bits are set, but the lower bits
-	 * match a valid pfn.
-	 */
-	if (PHYS_PFN(addr) != pfn)
-		return 0;
-
-#ifdef CONFIG_SPARSEMEM
-{
-	struct mem_section *ms;
-
-	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
-		return 0;
-
-	ms = __pfn_to_section(pfn);
-	if (!valid_section(ms))
-		return 0;
-
-	/*
-	 * ZONE_DEVICE memory does not have the memblock entries.
-	 * memblock_is_memory() check for ZONE_DEVICE based
-	 * addresses will always fail. Even the normal hotplugged
-	 * memory will never have MEMBLOCK_NOMAP flag set in their
-	 * memblock entries. Skip memblock search for all non early
-	 * memory sections covering all of hotplug memory including
-	 * both normal and ZONE_DEVICE based.
-	 */
-	if (!early_section(ms))
-		return pfn_section_valid(ms, pfn);
-}
-#endif
-	return memblock_is_memory(addr);
-}
-EXPORT_SYMBOL(pfn_valid);
-
 int pfn_is_map_memory(unsigned long pfn)
 {
 	phys_addr_t addr = PFN_PHYS(pfn);
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 961f0eeefb62..18bf71665211 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1421,10 +1421,22 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
  *
  * Return: 1 for PFNs that have memory map entries and 0 otherwise
  */
+bool memblock_is_memory(phys_addr_t addr);
+
 static inline int pfn_valid(unsigned long pfn)
 {
+	phys_addr_t addr = PFN_PHYS(pfn);
 	struct mem_section *ms;
 
+	/*
+	 * Ensure the upper PAGE_SHIFT bits are clear in the
+	 * pfn. Else it might lead to false positives when
+	 * some of the upper bits are set, but the lower bits
+	 * match a valid pfn.
+	 */
+	if (PHYS_PFN(addr) != pfn)
+		return 0;
+
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
 	ms = __nr_to_section(pfn_to_section_nr(pfn));
@@ -1434,7 +1446,11 @@ static inline int pfn_valid(unsigned long pfn)
 	 * Traditionally early sections always returned pfn_valid() for
 	 * the entire section-sized span.
 	 */
-	return early_section(ms) || pfn_section_valid(ms, pfn);
+	if (early_section(ms))
+		return IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) ?
+			memblock_is_memory(pfn << PAGE_SHIFT) : 1;
+
+	return pfn_section_valid(ms, pfn);
 }
 #endif
 
diff --git a/mm/Kconfig b/mm/Kconfig
index 24c045b24b95..db7128111874 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -135,6 +135,15 @@ config HAVE_FAST_GUP
 config ARCH_KEEP_MEMBLOCK
 	bool
 
+config HAVE_EARLY_SECTION_MEMMAP_HOLES
+	depends on ARCH_KEEP_MEMBLOCK && SPARSEMEM_VMEMMAP
+	def_bool n
+	help
+	  Early sections on certain platforms might have some memory ranges that
+	  are not backed with struct page mappings. When subscribed, this option
+	  enables special handling for those memory ranges in certain situations
+	  such as pfn_valid().
+
 # Keep arch NUMA mapping infrastructure post-init.
 config NUMA_KEEP_MEMINFO
 	bool
diff --git a/mm/memblock.c b/mm/memblock.c
index 3abf2c3fea7f..93f8a9c8428d 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1740,6 +1740,7 @@ bool __init_memblock memblock_is_memory(phys_addr_t addr)
 {
 	return memblock_search(&memblock.memory, addr) != -1;
 }
+EXPORT_SYMBOL(memblock_is_memory);
 
 bool __init_memblock memblock_is_map_memory(phys_addr_t addr)
 {
@@ -1931,8 +1932,11 @@ static void __init free_unused_memmap(void)
 	unsigned long start, end, prev_end = 0;
 	int i;
 
-	if (!IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) ||
-	    IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
+	if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP))
+		return;
+
+	if (!IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES) &&
+	    !IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID))
 		return;
 
 	/*
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0cdbbfbc5757..8c78b6a3d888 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -309,6 +309,11 @@ struct page *pfn_to_online_page(unsigned long pfn)
 	 * Save some code text when online_section() +
 	 * pfn_section_valid() are sufficient.
 	 */
+	if (IS_ENABLED(CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES)) {
+		if (early_section(ms) && !memblock_is_memory(PFN_PHYS(pfn)))
+			return NULL;
+	}
+
 	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
 		return NULL;
 
-- 
2.20.1