Re: [PATCH 6/7] mm,hwpoison: Disable pcplists before grabbing a refcount

Vlastimil Babka <vbabka@xxxxxxx> · Thu, 26 Nov 2020 14:45:30 +0100

On 11/19/20 11:57 AM, Oscar Salvador wrote:
Currently, we have a sort of retry mechanism to make sure pages in
pcp-lists are spilled to the buddy system, so we can handle those.

We can save us this extra checks with the new disable-pcplist mechanism
that is available with [1].

zone_pcplist_disable makes sure to 1) disable pcplists, so any page
that is freed up from that point onwards will end up in the buddy
system and 2) drain pcplists, so those pages that already in pcplists
are spilled to buddy.

With that, we can make a common entry point for grabbing a refcount
from both soft_offline and memory_failure paths that is guarded by
zone_pcplist_disable/zone_pcplist_enable.

[1] https://patchwork.kernel.org/project/linux-mm/cover/20201111092812.11329-1-vbabka@xxxxxxx/

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Note as you say the series should go after [1] above, which means after 
mm-page_alloc-disable-pcplists-during-memory-offline.patch in mmots, but 
currently it's ordered before, where zone_pcp_disable() etc doesn't yet exist. 
Found out as I review using checked out this commit from -next.

---
  mm/memory-failure.c | 120 +++++++++++++++++++++-----------------------
  1 file changed, 58 insertions(+), 62 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 4abf5d5ffc96..512613e9a1bd 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -985,26 +985,67 @@ static int __get_hwpoison_page(struct page *page)
  	return 0;
  }
  
-static int get_hwpoison_page(struct page *p)
+/*
+ * Safely get reference count of an arbitrary page.
+ *
+ * Returns 0 for a free page, 1 for an in-use page,
+ * -EIO for a page-type we cannot handle and -EBUSY if we raced with an
+ * allocation.
+ * We only incremented refcount in case the page was already in-use and it
+ * is a known type we can handle.
+ */
+static int get_any_page(struct page *p)
  {
-	int ret;
-	bool drained = false;
+	int ret = 0, pass = 0;
  
-retry:
-	ret = __get_hwpoison_page(p);
-	if (!ret && !is_free_buddy_page(p) && !page_count(p) && !drained) {
-		/*
-		 * The page might be in a pcplist, so try to drain those
-		 * and see if we are lucky.
-		 */
-		drain_all_pages(page_zone(p));
-		drained = true;
-		goto retry;
+try_again:
+	if (!__get_hwpoison_page(p)) {
+		if (page_count(p)) {
+			/* We raced with an allocation, retry. */
+			if (pass++ < 3)
+				goto try_again;
+			ret = -EBUSY;
+		} else if (!PageHuge(p) && !is_free_buddy_page(p)) {
+			/* We raced with put_page, retry. */
+			if (pass++ < 3)
+				goto try_again;
+			ret = -EIO;
+		}
+	} else {
+		if (PageHuge(p) || PageLRU(p) || __PageMovable(p)) {
+			ret = 1;
+		} else {
+			/*
+			 * A page we cannot handle. Check whether we can turn
+			 * it into something we can handle.
+			 */
+			if (pass++ < 3) {
+				put_page(p);
+				shake_page(p, 1);
+				goto try_again;
+			}
+			put_page(p);
+			ret = -EIO;
+		}
  	}
  
  	return ret;
  }
  
+static int get_hwpoison_page(struct page *p, enum mf_flags ctxt)
+{
+	int ret;
+
+	zone_pcp_disable(page_zone(p));
+	if (ctxt == MF_SOFT_OFFLINE)
+		ret = get_any_page(p);
+	else
+		ret = __get_hwpoison_page(p);
+	zone_pcp_enable(page_zone(p));
+
+	return ret;
+}
+
  /*
   * Do all that is necessary to remove user space mappings. Unmap
   * the pages and send SIGBUS to the processes if the data was dirty.
@@ -1185,7 +1226,7 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
  
  	num_poisoned_pages_inc();
  
-	if (!get_hwpoison_page(p)) {
+	if (!get_hwpoison_page(p, 0)) {
  		/*
  		 * Check "filter hit" and "race with other subpage."
  		 */
@@ -1387,7 +1428,7 @@ int memory_failure(unsigned long pfn, int flags)
  	 * In fact it's dangerous to directly bump up page count from 0,
  	 * that may make page_ref_freeze()/page_ref_unfreeze() mismatch.
  	 */
-	if (!get_hwpoison_page(p)) {
+	if (!get_hwpoison_page(p, 0)) {
  		if (is_free_buddy_page(p)) {
  			if (take_page_off_buddy(p)) {
  				page_ref_inc(p);
@@ -1674,7 +1715,7 @@ int unpoison_memory(unsigned long pfn)
  		return 0;
  	}
  
-	if (!get_hwpoison_page(p)) {
+	if (!get_hwpoison_page(p, 0)) {
  		if (TestClearPageHWPoison(p))
  			num_poisoned_pages_dec();
  		unpoison_pr_info("Unpoison: Software-unpoisoned free page %#lx\n",
@@ -1705,51 +1746,6 @@ int unpoison_memory(unsigned long pfn)
  }
  EXPORT_SYMBOL(unpoison_memory);
  
-/*
- * Safely get reference count of an arbitrary page.
- * Returns 0 for a free page, 1 for an in-use page, -EIO for a page-type we
- * cannot handle and -EBUSY if we raced with an allocation.
- * We only incremented refcount in case the page was already in-use and it is
- * a known type we can handle.
- */
-static int get_any_page(struct page *p)
-{
-	int ret = 0, pass = 0;
-
-try_again:
-	if (!get_hwpoison_page(p)) {
-		if (page_count(p)) {
-			/* We raced with an allocation, retry. */
-			if (pass++ < 3)
-				goto try_again;
-			ret = -EBUSY;
-		} else if (!PageHuge(p) && !is_free_buddy_page(p)) {
-			/* We raced with put_page, retry. */
-			if (pass++ < 3)
-				goto try_again;
-			ret = -EIO;
-		}
-	} else {
-		if (PageHuge(p) || PageLRU(p) || __PageMovable(p)) {
-			ret = 1;
-		} else {
-			/*
-			 * A page we cannot handle. Check whether we can turn
-			 * it into something we can handle.
-			 */
-			if (pass++ < 3) {
-				put_page(p);
-				shake_page(p, 1);
-				goto try_again;
-			}
-			put_page(p);
-			ret = -EIO;
-		}
-	}
-
-	return ret;
-}
-
  static bool isolate_page(struct page *page, struct list_head *pagelist)
  {
  	bool isolated = false;
@@ -1920,7 +1916,7 @@ int soft_offline_page(unsigned long pfn)
  
  retry:
  	get_online_mems();
-	ret = get_any_page(page);
+	ret = get_hwpoison_page(page, MF_SOFT_OFFLINE);
  	put_online_mems();
  
  	if (ret > 0) {









