[PATCH v9 1/2] mm/khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan <jiaqiyan@google.com>]

Make __collapse_huge_page_copy return whether copying anonymous pages
succeeded, and make collapse_huge_page handle the return status.

Break existing PTE scan loop into two for-loops. The first loop copies
source pages into target huge page, and can fail gracefully when running
into memory errors in source pages. If copying all pages succeeds, the
second loop releases and clears up these normal pages. Otherwise, the
second loop rolls back the page table and page states by:
- re-establishing the original PTEs-to-PMD connection.
- releasing source pages back to their LRU list.

Tested manually:
0. Enable khugepaged on system under test.
1. Start a two-thread application. Each thread allocates a chunk of
   non-huge anonymous memory buffer.
2. Pick 4 random buffer locations (2 in each thread) and inject
   uncorrectable memory errors at corresponding physical addresses.
3. Signal both threads to make their memory buffer collapsible, i.e.
   calling madvise(MADV_HUGEPAGE).
4. Wait and check kernel log: khugepaged is able to recover from poisoned
   pages and skips collapsing them.
5. Signal both threads to inspect their buffer contents and make sure no
   data corruption.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 179 ++++++++++++++++++++++-------
 2 files changed, 139 insertions(+), 43 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 35d759d3b0104..5743ae970af31 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -36,7 +36,8 @@
 	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
 	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\
 	EM( SCAN_TRUNCATED,		"truncated")			\
-	EMe(SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EMe(SCAN_COPY_MC,		"copy_poisoned_page")		\
 
 #undef EM
 #undef EMe
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5a7d2d5093f9c..0f1b9e05e17ec 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -19,6 +19,7 @@
 #include <linux/page_table_check.h>
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
+#include <linux/kmsan.h>
 
 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -55,6 +56,7 @@ enum scan_result {
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_COPY_MC,
 };
 
 #define CREATE_TRACE_POINTS
@@ -530,6 +532,27 @@ static bool is_refcount_suitable(struct page *page)
 	return page_count(page) == expected_refcount;
 }
 
+/*
+ * Copies memory with #MC in source page (@from) handled. Returns number
+ * of bytes not copied if there was an exception; otherwise 0 for success.
+ * Note handling #MC requires arch opt-in.
+ */
+static int copy_mc_page(struct page *to, struct page *from)
+{
+	char *vfrom, *vto;
+	unsigned long ret;
+
+	vfrom = kmap_local_page(from);
+	vto = kmap_local_page(to);
+	ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
+	if (ret == 0)
+		kmsan_copy_page_meta(to, from);
+	kunmap_local(vto);
+	kunmap_local(vfrom);
+
+	return ret;
+}
+
 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 					unsigned long address,
 					pte_t *pte,
@@ -670,56 +693,124 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	return result;
 }
 
-static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
-				      struct vm_area_struct *vma,
-				      unsigned long address,
-				      spinlock_t *ptl,
-				      struct list_head *compound_pagelist)
+/*
+ * __collapse_huge_page_copy - attempts to copy memory contents from normal
+ * pages to a hugepage. Cleans up the normal pages if copying succeeds;
+ * otherwise restores the original page table and releases isolated normal pages.
+ * Returns SCAN_SUCCEED if copying succeeds, otherwise returns SCAN_COPY_MC.
+ *
+ * @pte: starting of the PTEs to copy from
+ * @page: the new hugepage to copy contents to
+ * @pmd: pointer to the new hugepage's PMD
+ * @rollback: the original normal pages' PMD
+ * @vma: the original normal pages' virtual memory area
+ * @address: starting address to copy
+ * @pte_ptl: lock on normal pages' PTEs
+ * @compound_pagelist: list that stores compound pages
+ */
+static int __collapse_huge_page_copy(pte_t *pte,
+				     struct page *page,
+				     pmd_t *pmd,
+				     pmd_t rollback,
+				     struct vm_area_struct *vma,
+				     unsigned long address,
+				     spinlock_t *pte_ptl,
+				     struct list_head *compound_pagelist)
 {
 	struct page *src_page, *tmp;
 	pte_t *_pte;
-	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
-				_pte++, page++, address += PAGE_SIZE) {
-		pte_t pteval = *_pte;
+	pte_t pteval;
+	unsigned long _address;
+	spinlock_t *pmd_ptl;
+	int result = SCAN_SUCCEED;
 
-		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			clear_user_highpage(page, address);
-			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
-			if (is_zero_pfn(pte_pfn(pteval))) {
+	/*
+	 * Copying pages' contents is subject to memory poison at any iteration.
+	 */
+	for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, page++, _address += PAGE_SIZE) {
+		pteval = *_pte;
+
+		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
+			clear_user_highpage(page, _address);
+		else {
+			src_page = pte_page(pteval);
+			if (copy_mc_page(page, src_page) > 0) {
+				result = SCAN_COPY_MC;
+				break;
+			}
+		}
+	}
+
+	if (likely(result == SCAN_SUCCEED)) {
+		for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
+		     _pte++, _address += PAGE_SIZE) {
+			pteval = *_pte;
+			if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+				add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
+				if (is_zero_pfn(pte_pfn(pteval))) {
+					/*
+					 * pte_ptl mostly unnecessary.
+					 */
+					spin_lock(pte_ptl);
+					pte_clear(vma->vm_mm, _address, _pte);
+					spin_unlock(pte_ptl);
+				}
+			} else {
+				src_page = pte_page(pteval);
+				if (!PageCompound(src_page))
+					release_pte_page(src_page);
 				/*
-				 * ptl mostly unnecessary.
+				 * pte_ptl mostly unnecessary, but preempt has
+				 * to be disabled to update the per-cpu stats
+				 * inside page_remove_rmap().
 				 */
-				spin_lock(ptl);
-				ptep_clear(vma->vm_mm, address, _pte);
-				spin_unlock(ptl);
+				spin_lock(pte_ptl);
+				ptep_clear(vma->vm_mm, _address, _pte);
+				page_remove_rmap(src_page, vma, false);
+				spin_unlock(pte_ptl);
+				free_page_and_swap_cache(src_page);
+			}
+		}
+		list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
+			list_del(&src_page->lru);
+			mod_node_page_state(page_pgdat(src_page),
+					NR_ISOLATED_ANON + page_is_file_lru(src_page),
+					-compound_nr(src_page));
+			unlock_page(src_page);
+			free_swap_cache(src_page);
+			putback_lru_page(src_page);
+		}
+	} else {
+		/*
+		 * Re-establish the regular PMD that points to the regular
+		 * page table. Restoring PMD needs to be done prior to
+		 * releasing pages. Since pages are still isolated and
+		 * locked here, acquiring anon_vma_lock_write is unnecessary.
+		 */
+		pmd_ptl = pmd_lock(vma->vm_mm, pmd);
+		pmd_populate(vma->vm_mm, pmd, pmd_pgtable(rollback));
+		spin_unlock(pmd_ptl);
+		/*
+		 * Release both raw and compound pages isolated
+		 * in __collapse_huge_page_isolate.
+		 */
+		for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
+		     _pte++, _address += PAGE_SIZE) {
+			pteval = *_pte;
+			if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval))) {
+				src_page = pte_page(pteval);
+				if (!PageCompound(src_page))
+					release_pte_page(src_page);
 			}
-		} else {
-			src_page = pte_page(pteval);
-			copy_user_highpage(page, src_page, address, vma);
-			if (!PageCompound(src_page))
-				release_pte_page(src_page);
-			/*
-			 * ptl mostly unnecessary, but preempt has to
-			 * be disabled to update the per-cpu stats
-			 * inside page_remove_rmap().
-			 */
-			spin_lock(ptl);
-			ptep_clear(vma->vm_mm, address, _pte);
-			page_remove_rmap(src_page, vma, false);
-			spin_unlock(ptl);
-			free_page_and_swap_cache(src_page);
+		}
+		list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
+			list_del(&src_page->lru);
+			release_pte_page(src_page);
 		}
 	}
 
-	list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
-		list_del(&src_page->lru);
-		mod_node_page_state(page_pgdat(src_page),
-				    NR_ISOLATED_ANON + page_is_file_lru(src_page),
-				    -compound_nr(src_page));
-		unlock_page(src_page);
-		free_swap_cache(src_page);
-		putback_lru_page(src_page);
-	}
+	return result;
 }
 
 static void khugepaged_alloc_sleep(void)
@@ -1079,9 +1170,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 */
 	anon_vma_unlock_write(vma->anon_vma);
 
-	__collapse_huge_page_copy(pte, hpage, vma, address, pte_ptl,
-				  &compound_pagelist);
+	result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
+					   vma, address, pte_ptl,
+					   &compound_pagelist);
 	pte_unmap(pte);
+	if (unlikely(result != SCAN_SUCCEED))
+		goto out_up_write;
+
 	/*
 	 * spin_lock() below is not the equivalent of smp_wmb(), but
 	 * the smp_wmb() inside __SetPageUptodate() can be reused to
-- 
2.39.0.rc0.267.gcb52ba06e7-goog