On Thu 30 Nov 2017, 15:51, Michal Hocko wrote:

On Thu 23-11-17 11:14:38, Andrea Reale wrote:

quoted

When hot-removing memory we need to free vmemmap memory.
However, depending on the memory is being removed, it might
not be always possible to free a full vmemmap page / huge-page
because part of it might still be used.

Commit ae9aae9eda2d ("memory-hotplug: common APIs to support page tables
hot-remove") introduced a workaround for x86
hot-remove, by which partially unused areas are filled with
the 0xFD constant. Full pages are only removed when fully
filled by 0xFDs.

This commit introduces a MEMBLOCK_UNUSED_VMEMMAP memblock flag, with
the goal of using it in place of 0xFDs. For now, this will be used for
the arm64 port of memory hot remove, but the idea is to eventually use
the same mechanism for x86 as well.

Why cannot you use the same approach as x86 have? Have a look at the
vmemmap_free at al.

This arm64 hot-remove version (including vmemmap_free) is indeed an
almost 1-to-1 port of the x86 approach. 

If you look at the first version of the patchset we submitted a while 
ago (https://lkml.org/lkml/2017/4/11/540), we were initially using the
x86 approach of filling unsued page structs with 0xFDs. Commenting on
that, Mark suggested (and, indeed, I agree with him) that relying on a
magic constant for marking some portions of physical memory was quite
ugly. That is why we have used memblock for the purpose in this revised
patchset.

If you have a different view and any concrete suggestion on how to
improve this, it is definitely very well welcome. 

quoted

Signed-off-by: Andrea Reale <redacted>
Signed-off-by: Maciej Bielski <redacted>
---
 include/linux/memblock.h | 12 ++++++++++++
 mm/memblock.c            | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index bae11c7..0daec05 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h

@@ -26,6 +26,9 @@ enum {
 	MEMBLOCK_HOTPLUG	= 0x1,	/* hotpluggable region */
 	MEMBLOCK_MIRROR		= 0x2,	/* mirrored region */
 	MEMBLOCK_NOMAP		= 0x4,	/* don't add to kernel direct mapping */
+#ifdef CONFIG_MEMORY_HOTREMOVE
+	MEMBLOCK_UNUSED_VMEMMAP	= 0x8,  /* Mark VMEMAP blocks as dirty */
+#endif
 };
 
 struct memblock_region {

@@ -90,6 +93,10 @@ int memblock_mark_mirror(phys_addr_t base, phys_addr_t size);
 int memblock_mark_nomap(phys_addr_t base, phys_addr_t size);
 int memblock_clear_nomap(phys_addr_t base, phys_addr_t size);
 ulong choose_memblock_flags(void);
+#ifdef CONFIG_MEMORY_HOTREMOVE
+int memblock_mark_unused_vmemmap(phys_addr_t base, phys_addr_t size);
+int memblock_clear_unused_vmemmap(phys_addr_t base, phys_addr_t size);
+#endif
 
 /* Low level functions */
 int memblock_add_range(struct memblock_type *type,

@@ -182,6 +189,11 @@ static inline bool memblock_is_nomap(struct memblock_region *m)
 	return m->flags & MEMBLOCK_NOMAP;
 }
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+bool memblock_is_vmemmap_unused_range(struct memblock_type *mt,
+		phys_addr_t start, phys_addr_t end);
+#endif
+
 #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
 int memblock_search_pfn_nid(unsigned long pfn, unsigned long *start_pfn,
 			    unsigned long  *end_pfn);

diff --git a/mm/memblock.c b/mm/memblock.c
index 9120578..30d5aa4 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c

@@ -809,6 +809,18 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
 	return memblock_setclr_flag(base, size, 0, MEMBLOCK_NOMAP);
 }
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+int __init_memblock memblock_mark_unused_vmemmap(phys_addr_t base,
+		phys_addr_t size)
+{
+	return memblock_setclr_flag(base, size, 1, MEMBLOCK_UNUSED_VMEMMAP);
+}
+int __init_memblock memblock_clear_unused_vmemmap(phys_addr_t base,
+		phys_addr_t size)
+{
+	return memblock_setclr_flag(base, size, 0, MEMBLOCK_UNUSED_VMEMMAP);
+}
+#endif
 /**
  * __next_reserved_mem_region - next function for for_each_reserved_region()
  * @idx: pointer to u64 loop variable

@@ -1696,6 +1708,26 @@ void __init_memblock memblock_trim_memory(phys_addr_t align)
 	}
 }
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+bool __init_memblock memblock_is_vmemmap_unused_range(struct memblock_type *mt,
+		phys_addr_t start, phys_addr_t end)
+{
+	u64 i;
+	struct memblock_region *r;
+
+	i = memblock_search(mt, start);
+	r = &(mt->regions[i]);
+	while (r->base < end) {
+		if (!(r->flags & MEMBLOCK_UNUSED_VMEMMAP))
+			return 0;
+
+		r = &(memblock.memory.regions[++i]);
+	}
+
+	return 1;
+}
+#endif
+
 void __init_memblock memblock_set_current_limit(phys_addr_t limit)
 {
 	memblock.current_limit = limit;

-- 
2.7.4

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Thanks,
Andrea

-- 
Michal Hocko
SUSE Labs