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Subject[PATCH v9 08/12] mm: zero reserved and unavailable struct pages
Date
Some memory is reserved but unavailable: not present in memblock.memory
(because not backed by physical pages), but present in memblock.reserved.
Such memory has backing struct pages, but they are not initialized by going
through __init_single_page().

In some cases these struct pages are accessed even if they do not contain
any data. One example is page_to_pfn() might access page->flags if this is
where section information is stored (CONFIG_SPARSEMEM,
SECTION_IN_PAGE_FLAGS).

Since, struct pages are zeroed in __init_single_page(), and not during
allocation time, we must zero such struct pages explicitly.

The patch involves adding a new memblock iterator:
for_each_resv_unavail_range(i, p_start, p_end)

Which iterates through reserved && !memory lists, and we zero struct pages
explicitly by calling mm_zero_struct_page().

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
---
include/linux/memblock.h | 16 ++++++++++++++++
include/linux/mm.h | 6 ++++++
mm/page_alloc.c | 30 ++++++++++++++++++++++++++++++
3 files changed, 52 insertions(+)

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index bae11c7e7bf3..bdd4268f9323 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h
@@ -237,6 +237,22 @@ unsigned long memblock_next_valid_pfn(unsigned long pfn, unsigned long max_pfn);
for_each_mem_range_rev(i, &memblock.memory, &memblock.reserved, \
nid, flags, p_start, p_end, p_nid)

+/**
+ * for_each_resv_unavail_range - iterate through reserved and unavailable memory
+ * @i: u64 used as loop variable
+ * @flags: pick from blocks based on memory attributes
+ * @p_start: ptr to phys_addr_t for start address of the range, can be %NULL
+ * @p_end: ptr to phys_addr_t for end address of the range, can be %NULL
+ *
+ * Walks over unavailabled but reserved (reserved && !memory) areas of memblock.
+ * Available as soon as memblock is initialized.
+ * Note: because this memory does not belong to any physical node, flags and
+ * nid arguments do not make sense and thus not exported as arguments.
+ */
+#define for_each_resv_unavail_range(i, p_start, p_end) \
+ for_each_mem_range(i, &memblock.reserved, &memblock.memory, \
+ NUMA_NO_NODE, MEMBLOCK_NONE, p_start, p_end, NULL)
+
static inline void memblock_set_region_flags(struct memblock_region *r,
unsigned long flags)
{
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 50b74d628243..a7bba4ce79ba 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2010,6 +2010,12 @@ extern int __meminit __early_pfn_to_nid(unsigned long pfn,
struct mminit_pfnnid_cache *state);
#endif

+#ifdef CONFIG_HAVE_MEMBLOCK
+void zero_resv_unavail(void);
+#else
+static inline void zero_resv_unavail(void) {}
+#endif
+
extern void set_dma_reserve(unsigned long new_dma_reserve);
extern void memmap_init_zone(unsigned long, int, unsigned long,
unsigned long, enum memmap_context);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4b630ee91430..1d38d391dffd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6202,6 +6202,34 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
free_area_init_core(pgdat);
}

+#ifdef CONFIG_HAVE_MEMBLOCK
+/*
+ * Only struct pages that are backed by physical memory are zeroed and
+ * initialized by going through __init_single_page(). But, there are some
+ * struct pages which are reserved in memblock allocator and their fields
+ * may be accessed (for example page_to_pfn() on some configuration accesses
+ * flags). We must explicitly zero those struct pages.
+ */
+void __paginginit zero_resv_unavail(void)
+{
+ phys_addr_t start, end;
+ unsigned long pfn;
+ u64 i, pgcnt;
+
+ /* Loop through ranges that are reserved, but do not have reported
+ * physical memory backing.
+ */
+ pgcnt = 0;
+ for_each_resv_unavail_range(i, &start, &end) {
+ for (pfn = PFN_DOWN(start); pfn < PFN_UP(end); pfn++) {
+ mm_zero_struct_page(pfn_to_page(pfn));
+ pgcnt++;
+ }
+ }
+ pr_info("Reserved but unavailable: %lld pages", pgcnt);
+}
+#endif /* CONFIG_HAVE_MEMBLOCK */
+
#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP

#if MAX_NUMNODES > 1
@@ -6625,6 +6653,7 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)
node_set_state(nid, N_MEMORY);
check_for_memory(pgdat, nid);
}
+ zero_resv_unavail();
}

static int __init cmdline_parse_core(char *p, unsigned long *core)
@@ -6788,6 +6817,7 @@ void __init free_area_init(unsigned long *zones_size)
{
free_area_init_node(0, zones_size,
__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
+ zero_resv_unavail();
}

static int page_alloc_cpu_dead(unsigned int cpu)
--
2.14.1
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