[PATCH 6/6] mm/hwpoison-inject: keep the page online while filtering it

["Gregory Price (Meta)" <gourry@gourry.net>]

Injecting a memory error with the filter enabled while the memory is
hot-removed can crash.  hwpoison_inject() checks the pfn with
pfn_valid(), then runs shake_folio() and the filter on its page, which
can sleep in lru_add_drain_all():

  CPU 1 (hwpoison_inject)             CPU 2 (ACPI eject)
  pfn_valid()
  folio = page_folio(pfn_to_page())
                                      offline_pages()
                                      arch_remove_memory()
                                        vmemmap freed
  shake_folio(folio)
    reads folio->flags

  BUG: unable to handle page fault for address: ffd4000006040033
  #PF: error_code(0x0000) - not-present page
  RIP: 0010:shake_folio+0x2a/0x50
   hwpoison_inject+0x232/0x4b0 [hwpoison_inject]
   simple_attr_write_xsigned.constprop.0.isra.0+0x179/0x220
   debugfs_attr_write+0x63/0x90

Until the filter stopped pinning the page, that reference held off
offline.

Hold get_online_mems() around the filter, check the pfn with
pfn_to_online_page() under it, and drop the hold before
memory_failure(), which takes it again.  Offline and ZONE_DEVICE pages
never passed the filter anyway.  Export get/put_online_mems() to the
module.

With the injector held at shake_folio() on a free DIMM page across an
eject, base faults in 3 of 3 runs.  With this patch the eject waits, and
the injection then fails with -ENXIO, in 3 of 3.

Fixes: fd476720c9ba ("mm,hwpoison-inject: don't pin for hwpoison_filter")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/hwpoison-inject.c | 55 +++++++++++++++++++++++++++++---------------
 mm/memory_hotplug.c  |  2 ++
 2 files changed, 39 insertions(+), 18 deletions(-)

diff --git a/mm/hwpoison-inject.c b/mm/hwpoison-inject.c
index a11222572f978..f6cc967b3a468 100644
--- a/mm/hwpoison-inject.c
+++ b/mm/hwpoison-inject.c
@@ -9,6 +9,7 @@
 #include <linux/hugetlb.h>
 #include <linux/page-flags.h>
 #include <linux/memcontrol.h>
+#include <linux/memory_hotplug.h>
 #include "internal.h"
 
 static u32 hwpoison_filter_enable;
@@ -99,43 +100,61 @@ static int hwpoison_filter(struct page *p)
 
 static struct dentry *hwpoison_dir;
 
-static int hwpoison_inject(void *data, u64 val)
+/* Returns true if the filter rejects the page at @pfn */
+static bool hwpoison_inject_filtered(unsigned long pfn)
 {
-	unsigned long pfn = val;
 	struct page *p;
 	struct folio *folio;
-	int err;
-
-	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
-
-	if (!pfn_valid(pfn))
-		return -ENXIO;
 
-	p = pfn_to_page(pfn);
+	/*
+	 * Offline and ZONE_DEVICE pages are not LRU, hugetlb or free buddy
+	 * pages, so the checks below never let them through.
+	 */
+	p = pfn_to_online_page(pfn);
+	if (!p)
+		return true;
 	folio = page_folio(p);
 
-	if (!hwpoison_filter_enable)
-		goto inject;
-
 	shake_folio(folio);
 	/*
 	 * This implies unable to support non-LRU pages except free page.
 	 */
 	if (!folio_test_lru(folio) && !folio_test_hugetlb(folio) &&
 	    !is_free_buddy_page(p))
-		return 0;
+		return true;
 
 	/*
 	 * do a racy check to make sure PG_hwpoison will only be set for
 	 * the targeted owner (or on a free page).
 	 * memory_failure() will redo the check reliably inside page lock.
 	 */
-	err = hwpoison_filter(&folio->page);
-	if (err)
-		return 0;
+	return hwpoison_filter(&folio->page);
+}
+
+static int hwpoison_inject(void *data, u64 val)
+{
+	unsigned long pfn = val;
+	bool filtered;
+	int err;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (!pfn_valid(pfn))
+		return -ENXIO;
+
+	if (hwpoison_filter_enable) {
+		/*
+		 * Keep the page online while the filter looks at it.
+		 * memory_failure() takes this again, so drop it first.
+		 */
+		get_online_mems();
+		filtered = hwpoison_inject_filtered(pfn);
+		put_online_mems();
+		if (filtered)
+			return 0;
+	}
 
-inject:
 	pr_info("Injecting memory failure at pfn %#lx\n", pfn);
 	err = memory_failure(pfn, MF_SW_SIMULATED);
 	return (err == -EOPNOTSUPP) ? 0 : err;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b5d9e367b893d..fe2ed0894d49d 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -215,11 +215,13 @@ void get_online_mems(void)
 {
 	percpu_down_read(&mem_hotplug_lock);
 }
+EXPORT_SYMBOL_FOR_MODULES(get_online_mems, "hwpoison-inject");
 
 void put_online_mems(void)
 {
 	percpu_up_read(&mem_hotplug_lock);
 }
+EXPORT_SYMBOL_FOR_MODULES(put_online_mems, "hwpoison-inject");
 
 bool movable_node_enabled = false;
 
-- 
2.53.0-Meta