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Subject[RFC][PATCH 3/8] mm/vmscan: Attempt to migrate page in lieu of discard
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From: Dave Hansen <dave.hansen@linux.intel.com>

If a memory node has a preferred migration path to demote cold pages,
attempt to move those inactive pages to that migration node before
reclaiming. This will better utilize available memory, provide a faster
tier than swapping or discarding, and allow such pages to be reused
immediately without IO to retrieve the data.

When handling anonymous pages, this will be considered before swap if
enabled. Should the demotion fail for any reason, the page reclaim
will proceed as if the demotion feature was not enabled.

Some places we would like to see this used:

1. Persistent memory being as a slower, cheaper DRAM replacement
2. Remote memory-only "expansion" NUMA nodes
3. Resolving memory imbalances where one NUMA node is seeing more
allocation activity than another. This helps keep more recent
allocations closer to the CPUs on the node doing the allocating.

Yang Shi's patches used an alternative approach where to-be-discarded
pages were collected on a separate discard list and then discarded
as a batch with migrate_pages(). This results in simpler code and
has all the performance advantages of batching, but has the
disadvantage that pages which fail to migrate never get swapped.

#Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
---

b/include/linux/migrate.h | 6 ++++
b/include/trace/events/migrate.h | 3 +-
b/mm/debug.c | 1
b/mm/migrate.c | 52 +++++++++++++++++++++++++++++++++++++++
b/mm/vmscan.c | 25 ++++++++++++++++++
5 files changed, 86 insertions(+), 1 deletion(-)

diff -puN include/linux/migrate.h~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard include/linux/migrate.h
--- a/include/linux/migrate.h~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard 2020-06-29 16:34:38.950312604 -0700
+++ b/include/linux/migrate.h 2020-06-29 16:34:38.963312604 -0700
@@ -25,6 +25,7 @@ enum migrate_reason {
MR_MEMPOLICY_MBIND,
MR_NUMA_MISPLACED,
MR_CONTIG_RANGE,
+ MR_DEMOTION,
MR_TYPES
};

@@ -78,6 +79,7 @@ extern int migrate_huge_page_move_mappin
struct page *newpage, struct page *page);
extern int migrate_page_move_mapping(struct address_space *mapping,
struct page *newpage, struct page *page, int extra_count);
+extern int migrate_demote_mapping(struct page *page);
#else

static inline void putback_movable_pages(struct list_head *l) {}
@@ -104,6 +106,10 @@ static inline int migrate_huge_page_move
return -ENOSYS;
}

+static inline int migrate_demote_mapping(struct page *page)
+{
+ return -ENOSYS;
+}
#endif /* CONFIG_MIGRATION */

#ifdef CONFIG_COMPACTION
diff -puN include/trace/events/migrate.h~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard include/trace/events/migrate.h
--- a/include/trace/events/migrate.h~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard 2020-06-29 16:34:38.952312604 -0700
+++ b/include/trace/events/migrate.h 2020-06-29 16:34:38.963312604 -0700
@@ -20,7 +20,8 @@
EM( MR_SYSCALL, "syscall_or_cpuset") \
EM( MR_MEMPOLICY_MBIND, "mempolicy_mbind") \
EM( MR_NUMA_MISPLACED, "numa_misplaced") \
- EMe(MR_CONTIG_RANGE, "contig_range")
+ EM( MR_CONTIG_RANGE, "contig_range") \
+ EMe(MR_DEMOTION, "demotion")

/*
* First define the enums in the above macros to be exported to userspace
diff -puN mm/debug.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard mm/debug.c
--- a/mm/debug.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard 2020-06-29 16:34:38.954312604 -0700
+++ b/mm/debug.c 2020-06-29 16:34:38.963312604 -0700
@@ -25,6 +25,7 @@ const char *migrate_reason_names[MR_TYPE
"mempolicy_mbind",
"numa_misplaced",
"cma",
+ "demotion",
};

const struct trace_print_flags pageflag_names[] = {
diff -puN mm/migrate.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard mm/migrate.c
--- a/mm/migrate.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard 2020-06-29 16:34:38.956312604 -0700
+++ b/mm/migrate.c 2020-06-29 16:34:38.964312604 -0700
@@ -1151,6 +1151,58 @@ int next_demotion_node(int node)
return node;
}

+static struct page *alloc_demote_node_page(struct page *page, unsigned long node)
+{
+ /*
+ * 'mask' targets allocation only to the desired node in the
+ * migration path, and fails fast if the allocation can not be
+ * immediately satisfied. Reclaim is already active and heroic
+ * allocation efforts are unwanted.
+ */
+ gfp_t mask = GFP_NOWAIT | __GFP_NOWARN | __GFP_NORETRY |
+ __GFP_NOMEMALLOC | __GFP_THISNODE | __GFP_HIGHMEM |
+ __GFP_MOVABLE;
+ struct page *newpage;
+
+ if (PageTransHuge(page)) {
+ mask |= __GFP_COMP;
+ newpage = alloc_pages_node(node, mask, HPAGE_PMD_ORDER);
+ if (newpage)
+ prep_transhuge_page(newpage);
+ } else
+ newpage = alloc_pages_node(node, mask, 0);
+
+ return newpage;
+}
+
+/**
+ * migrate_demote_mapping() - Migrate this page and its mappings to its
+ * demotion node.
+ * @page: A locked, isolated, non-huge page that should migrate to its current
+ * node's demotion target, if available. Since this is intended to be
+ * called during memory reclaim, all flag options are set to fail fast.
+ *
+ * @returns: MIGRATEPAGE_SUCCESS if successful, -errno otherwise.
+ */
+int migrate_demote_mapping(struct page *page)
+{
+ int next_nid = next_demotion_node(page_to_nid(page));
+
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+ VM_BUG_ON_PAGE(PageHuge(page), page);
+ VM_BUG_ON_PAGE(PageLRU(page), page);
+
+ if (next_nid == NUMA_NO_NODE)
+ return -ENOSYS;
+ if (PageTransHuge(page) && !thp_migration_supported())
+ return -ENOMEM;
+
+ /* MIGRATE_ASYNC is the most light weight and never blocks.*/
+ return __unmap_and_move(alloc_demote_node_page, NULL, next_nid,
+ page, MIGRATE_ASYNC, MR_DEMOTION);
+}
+
+
/*
* gcc 4.7 and 4.8 on arm get an ICEs when inlining unmap_and_move(). Work
* around it.
diff -puN mm/vmscan.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard mm/vmscan.c
--- a/mm/vmscan.c~0008-mm-vmscan-Attempt-to-migrate-page-in-lieu-of-discard 2020-06-29 16:34:38.959312604 -0700
+++ b/mm/vmscan.c 2020-06-29 16:34:38.965312604 -0700
@@ -1077,6 +1077,7 @@ static unsigned long shrink_page_list(st
LIST_HEAD(free_pages);
unsigned nr_reclaimed = 0;
unsigned pgactivate = 0;
+ int rc;

memset(stat, 0, sizeof(*stat));
cond_resched();
@@ -1229,6 +1230,30 @@ static unsigned long shrink_page_list(st
; /* try to reclaim the page below */
}

+ rc = migrate_demote_mapping(page);
+ /*
+ * -ENOMEM on a THP may indicate either migration is
+ * unsupported or there was not enough contiguous
+ * space. Split the THP into base pages and retry the
+ * head immediately. The tail pages will be considered
+ * individually within the current loop's page list.
+ */
+ if (rc == -ENOMEM && PageTransHuge(page) &&
+ !split_huge_page_to_list(page, page_list))
+ rc = migrate_demote_mapping(page);
+
+ if (rc == MIGRATEPAGE_SUCCESS) {
+ unlock_page(page);
+ if (likely(put_page_testzero(page)))
+ goto free_it;
+ /*
+ * Speculative reference will free this page,
+ * so leave it off the LRU.
+ */
+ nr_reclaimed++;
+ continue;
+ }
+
/*
* Anonymous process memory has backing store?
* Try to allocate it some swap space here.
_
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