Re: your mail

Michal Hocko
2017-04-20T08:49:43Z

On Thu 20-04-17 09:28:20, Michal Hocko wrote:
> On Thu 20-04-17 10:27:55, Joonsoo Kim wrote:
[...]
> > Your patch try to add PageReserved() to __pageblock_pfn_to_page(). It
> > woule make that zone->contiguous usually returns false since memory
> > used by memblock API is marked as PageReserved() and your patch regard
> > it as a hole. It invalidates set_zone_contiguous() optimization and I
> > worry about it.
>
> OK, fair enough. I did't consider memblock allocations. I will rethink
> this patch but there are essentially 3 options
> - use a different criterion for the offline holes dection. I
> have just realized we might do it by storing the online
> information into the mem sections
> - drop this patch
> - move the PageReferenced check down the chain into
> isolate_freepages_block resp. isolate_migratepages_block
>
> I would prefer 3 over 2 over 1. I definitely want to make this more
> robust so 1 is preferable long term but I do not want this to be a
> roadblock to the rest of the rework. Does that sound acceptable to you?

So I've played with all three options just to see how the outcome would
look like and it turned out that going with 1 will be easiest in the
end. What do you think about the following? It should be free of any
false positives. I have only compile tested it yet.
---
>From 747794c13c0e82b55b793a31cdbe1a84ee1c6920 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Thu, 13 Apr 2017 10:28:45 +0200
Subject: [PATCH] mm: consider zone which is not fully populated to have holes

__pageblock_pfn_to_page has two users currently, set_zone_contiguous
which checks whether the given zone contains holes and
pageblock_pfn_to_page which then carefully returns a first valid
page from the given pfn range for the given zone. This doesn't handle
zones which are not fully populated though. Memory pageblocks can be
offlined or might not have been onlined yet. In such a case the zone
should be considered to have holes otherwise pfn walkers can touch
and play with offline pages.

Current callers of pageblock_pfn_to_page in compaction seem to work
properly right now because they only isolate PageBuddy
(isolate_freepages_block) or PageLRU resp. __PageMovable
(isolate_migratepages_block) which will be always false for these pages.
It would be safer to skip these pages altogether, though.

In order to do this patch adds a new memory section state
(SECTION_IS_ONLINE) which is set in memory_present (during boot
time) or in online_pages_range during the memory hotplug. Similarly
offline_mem_sections clears the bit and it is called when the memory
range is offlined.

pfn_to_online_page helper is then added which check the mem section and
only returns a page if it is onlined already.

Use the new helper in __pageblock_pfn_to_page and skip the whole page
block in such a case.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
include/linux/memory_hotplug.h | 21 ++++++++++++++++++++
include/linux/mmzone.h | 20 ++++++++++++++++++-
mm/memory_hotplug.c | 3 +++
mm/page_alloc.c | 5 ++++-
mm/sparse.c | 45 +++++++++++++++++++++++++++++++++++++++++-
5 files changed, 91 insertions(+), 3 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 3c8cf86201c3..fc1c873504eb 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -14,6 +14,19 @@ struct memory_block;
struct resource;

#ifdef CONFIG_MEMORY_HOTPLUG
+/*
+ * Return page for the valid pfn only if the page is online. All pfn
+ * walkers which rely on the fully initialized page->flags and others
+ * should use this rather than pfn_valid && pfn_to_page
+ */
+#define pfn_to_online_page(pfn) \
+({ \
+ struct page *___page = NULL; \
+ \
+ if (online_section_nr(pfn_to_section_nr(pfn))) \
+ ___page = pfn_to_page(pfn); \
+ ___page; \
+})

/*
* Types for free bootmem stored in page->lru.next. These have to be in
@@ -203,6 +216,14 @@ extern void set_zone_contiguous(struct zone *zone);
extern void clear_zone_contiguous(struct zone *zone);

#else /* ! CONFIG_MEMORY_HOTPLUG */
+#define pfn_to_online_page(pfn) \
+({ \
+ struct page *___page = NULL; \
+ if (pfn_valid(pfn)) \
+ ___page = pfn_to_page(pfn); \
+ ___page; \
+ })
+
/*
* Stub functions for when hotplug is off
*/
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0fc121bbf4ff..cad16ac080f5 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1143,7 +1143,8 @@ extern unsigned long usemap_size(void);
*/
#define SECTION_MARKED_PRESENT (1UL<<0)
#define SECTION_HAS_MEM_MAP (1UL<<1)
-#define SECTION_MAP_LAST_BIT (1UL<<2)
+#define SECTION_IS_ONLINE (1UL<<2)
+#define SECTION_MAP_LAST_BIT (1UL<<3)
#define SECTION_MAP_MASK (~(SECTION_MAP_LAST_BIT-1))
#define SECTION_NID_SHIFT 2

@@ -1174,6 +1175,23 @@ static inline int valid_section_nr(unsigned long nr)
return valid_section(__nr_to_section(nr));
}

+static inline int online_section(struct mem_section *section)
+{
+ return (section && (section->section_mem_map & SECTION_IS_ONLINE));
+}
+
+static inline int online_section_nr(unsigned long nr)
+{
+ return online_section(__nr_to_section(nr));
+}
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
+#ifdef CONFIG_MEMORY_HOTREMOVE
+void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn);
+#endif
+#endif
+
static inline struct mem_section *__pfn_to_section(unsigned long pfn)
{
return __nr_to_section(pfn_to_section_nr(pfn));
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index caa58338d121..98f565c279bf 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -929,6 +929,9 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
unsigned long i;
unsigned long onlined_pages = *(unsigned long *)arg;
struct page *page;
+
+ online_mem_sections(start_pfn, start_pfn + nr_pages);
+
if (PageReserved(pfn_to_page(start_pfn)))
for (i = 0; i < nr_pages; i++) {
page = pfn_to_page(start_pfn + i);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5d72d29a6ece..fa752de84eef 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1353,7 +1353,9 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
return NULL;

- start_page = pfn_to_page(start_pfn);
+ start_page = pfn_to_online_page(start_pfn);
+ if (!start_page)
+ return NULL;

if (page_zone(start_page) != zone)
return NULL;
@@ -7686,6 +7688,7 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
break;
if (pfn == end_pfn)
return;
+ offline_mem_sections(pfn, end_pfn);
zone = page_zone(pfn_to_page(pfn));
spin_lock_irqsave(&zone->lock, flags);
pfn = start_pfn;
diff --git a/mm/sparse.c b/mm/sparse.c
index 6903c8fc3085..79017f90d8fc 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -185,7 +185,8 @@ void __init memory_present(int nid, unsigned long start, unsigned long end)
ms = __nr_to_section(section);
if (!ms->section_mem_map)
ms->section_mem_map = sparse_encode_early_nid(nid) |
- SECTION_MARKED_PRESENT;
+ SECTION_MARKED_PRESENT |
+ SECTION_IS_ONLINE;
}
}

@@ -590,6 +591,48 @@ void __init sparse_init(void)
}

#ifdef CONFIG_MEMORY_HOTPLUG
+
+/* Mark all memory sections within the pfn range as online */
+void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
+{
+ unsigned long pfn;
+
+ for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+ unsigned long section_nr = pfn_to_section_nr(start_pfn);
+ struct mem_section *ms;
+
+ /* onlining code should never touch invalid ranges */
+ if (WARN_ON(!valid_section_nr(section_nr)))
+ continue;
+
+ ms = __nr_to_section(section_nr);
+ ms->section_mem_map |= SECTION_IS_ONLINE;
+ }
+}
+
+#ifdef CONFIG_MEMORY_HOTREMOVE
+/* Mark all memory sections within the pfn range as online */
+void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
+{
+ unsigned long pfn;
+
+ for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+ unsigned long section_nr = pfn_to_section_nr(start_pfn);
+ struct mem_section *ms;
+
+ /*
+ * TODO this needs some double checking. Offlining code makes
+ * sure to check pfn_valid but those checks might be just bogus
+ */
+ if (WARN_ON(!valid_section_nr(section_nr)))
+ continue;
+
+ ms = __nr_to_section(section_nr);
+ ms->section_mem_map &= ~SECTION_IS_ONLINE;
+ }
+}
+#endif
+
#ifdef CONFIG_SPARSEMEM_VMEMMAP
static inline struct page *kmalloc_section_memmap(unsigned long pnum, int nid)
{
--
2.11.0

--
Michal Hocko
SUSE Labs