On 04.01.21 07:18, Anshuman Khandual wrote:

On 12/22/20 2:41 PM, David Hildenbrand wrote:

quoted

On 22.12.20 08:12, Anshuman Khandual wrote:

quoted

pfn_valid() validates a pfn but basically it checks for a valid struct page
backing for that pfn. It should always return positive for memory ranges
backed with struct page mapping. But currently pfn_valid() fails for all
ZONE_DEVICE based memory types even though they have struct page mapping.

pfn_valid() asserts that there is a memblock entry for a given pfn without
MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
that they do not have memblock entries. Hence memblock_is_map_memory() will
invariably fail via memblock_search() for a ZONE_DEVICE based address. This
eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
into the system via memremap_pages() called from a driver, their respective
memory sections will not have SECTION_IS_EARLY set.

Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
for firmware reserved memory regions. memblock_is_map_memory() can just be
skipped as its always going to be positive and that will be an optimization
for the normal hotplug memory. Like ZONE_DEVIE based memory, all hotplugged
normal memory too will not have SECTION_IS_EARLY set for their sections.

Skipping memblock_is_map_memory() for all non early memory sections would
fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
performance for normal hotplug memory as well.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
Signed-off-by: Anshuman Khandual <redacted>
---
 arch/arm64/mm/init.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 75addb36354a..ee23bda00c28 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -225,6 +225,18 @@ int pfn_valid(unsigned long pfn)
 
 	if (!valid_section(__pfn_to_section(pfn)))
 		return 0;
+
+	/*
+	 * ZONE_DEVICE memory does not have the memblock entries.
+	 * memblock_is_map_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_DEVIE based.
+	 */
+	if (!early_section(__pfn_to_section(pfn)))
+		return 1;

Actually, I think we want to check for partial present sections.

Maybe we can rather switch to generic pfn_valid() and tweak it to
something like

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index fb3bf696c05e..7b1fcce5bd5a 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -1382,9 +1382,13 @@ static inline int pfn_valid(unsigned long pfn)
                return 0;
        /*
         * Traditionally early sections always returned pfn_valid() for
-        * the entire section-sized span.
+        * the entire section-sized span. Some archs might have holes in
+        * early sections, so double check with memblock if configured.
         */
-       return early_section(ms) || pfn_section_valid(ms, pfn);
+       if (early_section(ms))
+               return IS_ENABLED(CONFIG_EARLY_SECTION_MEMMAP_HOLES) ?
+                      memblock_is_map_memory(pfn << PAGE_SHIFT) : 1;
+       return pfn_section_valid(ms, pfn);
 }
 #endif

Could not find CONFIG_EARLY_SECTION_MEMMAP_HOLES. Are you suggesting to
create this config which could track platform scenarios where all early
sections might not have mmap coverage such as arm64 ?

Yes, a new config that states what's actually happening.

quoted

Which users are remaining that require us to add/remove memblocks when
hot(un)plugging memory

 $ git grep KEEP_MEM | grep memory_hotplug
mm/memory_hotplug.c:    if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK))
mm/memory_hotplug.c:    if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK))
mm/memory_hotplug.c:    if (IS_ENABLED(CONFIG_ARCH_KEEP_MEMBLOCK)) {

Did not follow, do we want to drop ARCH_KEEP_MEMBLOCK ? Without it arm64
will not be able to track MEMBLOCK_NOMAP memory at runtime.

I'd only like the hot(un)plug parts gone for now, if possible: I don't
see the need for that handling really that cannot be handled easier, as
in the proposed pfn_valid() changes.

I understand that current handling of memory holes in early sections and
memory marked as MEMBLOCK_NOMAP requires ARCH_KEEP_MEMBLOCK for now.

quoted

I think one user we would have to handle is
arch/arm64/mm/mmap.c:valid_phys_addr_range(). AFAIS, powerpc at least
does not rely on memblock_is_map_memory.

memblock_is_map_memory() is currently used only on arm/arm64 platforms.
Apart from the above example in valid_phys_addr_range(), there are some
other memblock_is_map_memory() call sites as well. But then, we are not
trying to completely drop memblock_is_map_memory() or are we ?

No, just change the semantics: only relevant for early sections. Imagine
freezing MEMBLOCK state after boot.

Only early sections can have memory holes and might be marked
MEMBLOCK_NOMAP. For hotplugged memory, we don't have to call
memblock_is_map_memory().

-- 
Thanks,

David / dhildenb


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel