HWPOISON: Add unpoisoning support

The unpoisoning interface is useful for stress testing tools to
reclaim poisoned pages (to prevent OOM)

There is no hardware level unpoisioning, so this
cannot be used for real memory errors, only for software injected errors.

Note that it may leak pages silently - those who have been removed from
LRU cache, but not isolated from page cache/swap cache at hwpoison time.
Especially the stress test of dirty swap cache pages shall reboot system
before exhausting memory.

AK: Fix comments, add documentation, add printks, rename symbol

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

2 files changed, 98 insertions, 6 deletions

diff --git a/mm/hwpoison-inject.c b/mm/hwpoison-inject.c
index e1d85137f086..6e35e563bf50 100644
--- a/mm/hwpoison-inject.c
+++ b/mm/hwpoison-inject.c
@@ -4,7 +4,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 
-static struct dentry *hwpoison_dir, *corrupt_pfn;
+static struct dentry *hwpoison_dir;
 
 static int hwpoison_inject(void *data, u64 val)
 {
@@ -14,7 +14,16 @@ static int hwpoison_inject(void *data, u64 val)
         return __memory_failure(val, 18, 0);
 }
 
+static int hwpoison_unpoison(void *data, u64 val)
+{
+        if (!capable(CAP_SYS_ADMIN))
+                return -EPERM;
+
+        return unpoison_memory(val);
+}
+
 DEFINE_SIMPLE_ATTRIBUTE(hwpoison_fops, NULL, hwpoison_inject, "%lli\n");
+DEFINE_SIMPLE_ATTRIBUTE(unpoison_fops, NULL, hwpoison_unpoison, "%lli\n");
 
 static void pfn_inject_exit(void)
 {
@@ -24,16 +33,31 @@ static void pfn_inject_exit(void)
 
 static int pfn_inject_init(void)
 {
+        struct dentry *dentry;
+
         hwpoison_dir = debugfs_create_dir("hwpoison", NULL);
         if (hwpoison_dir == NULL)
                 return -ENOMEM;
-        corrupt_pfn = debugfs_create_file("corrupt-pfn", 0600, hwpoison_dir,
+
+        /*
+         * Note that the below poison/unpoison interfaces do not involve
+         * hardware status change, hence do not require hardware support.
+         * They are mainly for testing hwpoison in software level.
+         */
+        dentry = debugfs_create_file("corrupt-pfn", 0600, hwpoison_dir,
                                           NULL, &hwpoison_fops);
-        if (corrupt_pfn == NULL) {
-                pfn_inject_exit();
-                return -ENOMEM;
-        }
+        if (!dentry)
+                goto fail;
+
+        dentry = debugfs_create_file("unpoison-pfn", 0600, hwpoison_dir,
+                                     NULL, &unpoison_fops);
+        if (!dentry)
+                goto fail;
+
         return 0;
+fail:
+        pfn_inject_exit();
+        return -ENOMEM;
 }
 
 module_init(pfn_inject_init);
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 5055b940df5f..ed6e91c87a54 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -838,6 +838,16 @@ int __memory_failure(unsigned long pfn, int trapno, int flags)
          * and in many cases impossible, so we just avoid it here.
          */
         lock_page_nosync(p);
+
+        /*
+         * unpoison always clear PG_hwpoison inside page lock
+         */
+        if (!PageHWPoison(p)) {
+                action_result(pfn, "unpoisoned", IGNORED);
+                res = 0;
+                goto out;
+        }
+
         wait_on_page_writeback(p);
 
         /*
@@ -893,3 +903,61 @@ void memory_failure(unsigned long pfn, int trapno)
 {
         __memory_failure(pfn, trapno, 0);
 }
+
+/**
+ * unpoison_memory - Unpoison a previously poisoned page
+ * @pfn: Page number of the to be unpoisoned page
+ *
+ * Software-unpoison a page that has been poisoned by
+ * memory_failure() earlier.
+ *
+ * This is only done on the software-level, so it only works
+ * for linux injected failures, not real hardware failures
+ *
+ * Returns 0 for success, otherwise -errno.
+ */
+int unpoison_memory(unsigned long pfn)
+{
+        struct page *page;
+        struct page *p;
+        int freeit = 0;
+
+        if (!pfn_valid(pfn))
+                return -ENXIO;
+
+        p = pfn_to_page(pfn);
+        page = compound_head(p);
+
+        if (!PageHWPoison(p)) {
+                pr_debug("MCE: Page was already unpoisoned %#lx\n", pfn);
+                return 0;
+        }
+
+        if (!get_page_unless_zero(page)) {
+                if (TestClearPageHWPoison(p))
+                        atomic_long_dec(&mce_bad_pages);
+                pr_debug("MCE: Software-unpoisoned free page %#lx\n", pfn);
+                return 0;
+        }
+
+        lock_page_nosync(page);
+        /*
+         * This test is racy because PG_hwpoison is set outside of page lock.
+         * That's acceptable because that won't trigger kernel panic. Instead,
+         * the PG_hwpoison page will be caught and isolated on the entrance to
+         * the free buddy page pool.
+         */
+        if (TestClearPageHWPoison(p)) {
+                pr_debug("MCE: Software-unpoisoned page %#lx\n", pfn);
+                atomic_long_dec(&mce_bad_pages);
+                freeit = 1;
+        }
+        unlock_page(page);
+
+        put_page(page);
+        if (freeit)
+                put_page(page);
+
+        return 0;
+}
+EXPORT_SYMBOL(unpoison_memory);