[PATCH v5 3/4] x86/vmemmap: Handle unpopulated sub-pmd ranges

Oscar Salvador
2021-03-09T17:41:32Z

When sizeof(struct page) is not a power of 2, sections do not span
a PMD anymore and so when populating them some parts of the PMD will
remain unused.
Because of this, PMDs will be left behind when depopulating sections
since remove_pmd_table() thinks that those unused parts are still in
use.

Fix this by marking the unused parts with PAGE_UNUSED, so memchr_inv()
will do the right thing and will let us free the PMD when the last user
of it is gone.

This patch is based on a similar patch by David Hildenbrand:

https://lore.kernel.org/linux-mm/20200722094558.9828-9-david@redhat.com/

Signed-off-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
arch/x86/mm/init_64.c | 63 ++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 52 insertions(+), 11 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 9ecb3c488ac8..3bb3988c7681 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -871,7 +871,50 @@ int arch_add_memory(int nid, u64 start, u64 size,
return add_pages(nid, start_pfn, nr_pages, params);
}

-#define PAGE_INUSE 0xFD
+#ifdef CONFIG_SPARSEMEM_VMEMMAP
+#define PAGE_UNUSED 0xFD
+
+/* Returns true if the PMD is completely unused and thus it can be freed */
+static bool __meminit vmemmap_pmd_is_unused(unsigned long addr, unsigned long end)
+{
+ unsigned long start = ALIGN_DOWN(addr, PMD_SIZE);
+
+ memset((void *)addr, PAGE_UNUSED, end - addr);
+
+ return !memchr_inv((void *)start, PAGE_UNUSED, PMD_SIZE);
+}
+
+static void __meminit vmemmap_use_sub_pmd(unsigned long start)
+{
+ /*
+ * As we expect to add in the same granularity as we remove, it's
+ * sufficient to mark only some piece used to block the memmap page from
+ * getting removed when removing some other adjacent memmap (just in
+ * case the first memmap never gets initialized e.g., because the memory
+ * block never gets onlined).
+ */
+ memset((void *)start, 0, sizeof(struct page));
+}
+
+static void __meminit vmemmap_use_new_sub_pmd(unsigned long start, unsigned long end)
+{
+ /*
+ * Could be our memmap page is filled with PAGE_UNUSED already from a
+ * previous remove. Make sure to reset it.
+ */
+ vmemmap_use_sub_pmd(start);
+
+ /*
+ * Mark with PAGE_UNUSED the unused parts of the new memmap range
+ */
+ if (!IS_ALIGNED(start, PMD_SIZE))
+ memset((void *)start, PAGE_UNUSED,
+ start - ALIGN_DOWN(start, PMD_SIZE));
+ if (!IS_ALIGNED(end, PMD_SIZE))
+ memset((void *)end, PAGE_UNUSED,
+ ALIGN(end, PMD_SIZE) - end);
+}
+#endif

static void __meminit free_pagetable(struct page *page, int order)
{
@@ -1006,7 +1049,6 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
unsigned long next, pages = 0;
pte_t *pte_base;
pmd_t *pmd;
- void *page_addr;

pmd = pmd_start + pmd_index(addr);
for (; addr < end; addr = next, pmd++) {
@@ -1026,20 +1068,13 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
pmd_clear(pmd);
spin_unlock(&init_mm.page_table_lock);
pages++;
- } else {
- /* If here, we are freeing vmemmap pages. */
- memset((void *)addr, PAGE_INUSE, next - addr);
-
- page_addr = page_address(pmd_page(*pmd));
- if (!memchr_inv(page_addr, PAGE_INUSE,
- PMD_SIZE)) {
+ } else if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
+ vmemmap_pmd_is_unused(addr, next)) {
free_hugepage_table(pmd_page(*pmd),
altmap);
-
spin_lock(&init_mm.page_table_lock);
pmd_clear(pmd);
spin_unlock(&init_mm.page_table_lock);
- }
}

continue;
@@ -1492,11 +1527,17 @@ static int __meminit vmemmap_populate_hugepages(unsigned long start,

addr_end = addr + PMD_SIZE;
p_end = p + PMD_SIZE;
+
+ if (!IS_ALIGNED(addr, PMD_SIZE) ||
+ !IS_ALIGNED(next, PMD_SIZE))
+ vmemmap_use_new_sub_pmd(addr, next);
+
continue;
} else if (altmap)
return -ENOMEM; /* no fallback */
} else if (pmd_large(*pmd)) {
vmemmap_verify((pte_t *)pmd, node, addr, next);
+ vmemmap_use_sub_pmd(addr);
continue;
}
if (vmemmap_populate_basepages(addr, next, node, NULL))
--
2.16.3