Re: [PATCH] mm,hwpoison: return -EBUSY when page already poisoned

["Luck, Tony" <tony.luck@xxxxxxxxx>]

This whole page table walking patch is trying to work around the
races caused by multiple calls to memory_failure() for the same
page.

Maybe better to just avoid the races.  The comment right above
memory_failure says:

 * Must run in process context (e.g. a work queue) with interrupts
 * enabled and no spinlocks hold.

So it should be safe to grab and hold a mutex.  See patch below.

-Tony

commit 8dd0dbe7d595e02647e9c2c76c03341a9f6bd7b9
Author: Tony Luck <tony.luck@xxxxxxxxx>
Date:   Fri Mar 5 10:40:48 2021 -0800

    Use a mutex to avoid memory_failure() races

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 24210c9bd843..c1509f4b565e 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1381,6 +1381,8 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
 	return rc;
 }
 
+static DEFINE_MUTEX(mf_mutex);
+
 /**
  * memory_failure - Handle memory failure of a page.
  * @pfn: Page Number of the corrupted page
@@ -1424,12 +1426,18 @@ int memory_failure(unsigned long pfn, int flags)
 		return -ENXIO;
 	}
 
+	mutex_lock(&mf_mutex);
+
 try_again:
-	if (PageHuge(p))
-		return memory_failure_hugetlb(pfn, flags);
+	if (PageHuge(p)) {
+		res = memory_failure_hugetlb(pfn, flags);
+		goto out2;
+	}
+
 	if (TestSetPageHWPoison(p)) {
 		pr_err("Memory failure: %#lx: already hardware poisoned\n",
 			pfn);
+		mutex_unlock(&mf_mutex);
 		return 0;
 	}
 
@@ -1463,9 +1471,11 @@ int memory_failure(unsigned long pfn, int flags)
 				res = MF_FAILED;
 			}
 			action_result(pfn, MF_MSG_BUDDY, res);
+			mutex_unlock(&mf_mutex);
 			return res == MF_RECOVERED ? 0 : -EBUSY;
 		} else {
 			action_result(pfn, MF_MSG_KERNEL_HIGH_ORDER, MF_IGNORED);
+			mutex_unlock(&mf_mutex);
 			return -EBUSY;
 		}
 	}
@@ -1473,6 +1483,7 @@ int memory_failure(unsigned long pfn, int flags)
 	if (PageTransHuge(hpage)) {
 		if (try_to_split_thp_page(p, "Memory Failure") < 0) {
 			action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
+			mutex_unlock(&mf_mutex);
 			return -EBUSY;
 		}
 		VM_BUG_ON_PAGE(!page_count(p), p);
@@ -1517,6 +1528,7 @@ int memory_failure(unsigned long pfn, int flags)
 		num_poisoned_pages_dec();
 		unlock_page(p);
 		put_page(p);
+		mutex_unlock(&mf_mutex);
 		return 0;
 	}
 	if (hwpoison_filter(p)) {
@@ -1524,6 +1536,7 @@ int memory_failure(unsigned long pfn, int flags)
 			num_poisoned_pages_dec();
 		unlock_page(p);
 		put_page(p);
+		mutex_unlock(&mf_mutex);
 		return 0;
 	}
 
@@ -1559,6 +1572,8 @@ int memory_failure(unsigned long pfn, int flags)
 	res = identify_page_state(pfn, p, page_flags);
 out:
 	unlock_page(p);
+out2:
+	mutex_unlock(&mf_mutex);
 	return res;
 }
 EXPORT_SYMBOL_GPL(memory_failure);