[RFC v2 0/2] Memory poison recovery in khugepaged collapsing

[RFC v2 0/2] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors.
As memory size and density increase, the chances of and number of
memory errors increase. The increasing size and density of server
RAM in the data center and cloud have shown increased uncorrectable
memory errors. There are already mechanisms in the kernel to recover
from uncorrectable memory errors. This series of patches provides
the recovery mechanism for the particular kernel agent khugepaged
when it collapses memory pages.

Impact
======
The main reason we chose to make khugepaged collapsing tolerant of
memory failures was its high possibility of accessing poisoned memory
while performing functionally optional compaction actions.
Standard applications typically don't have strict requirements on
the size of its pages. So they are given 4K pages by the kernel.
The kernel is able to improve application performance by either

  1) giving application 2M pages to begin with, or
  2) collapsing 4K pages into 2M pages when possible.

This collapsing operation is done by khugepaged, a kernel agent that
is constantly scanning memory. When collapsing 4K pages into a 2M page,
it must copy the data from the 4K pages into a physically contiguous
2M page. Therefore, as long as there exists one poisoned cache line in
collapsible 4K pages, khugepaged will eventually access it. The current
impact to users is a machine check exception triggered kernel panic.
However, khugepaged’s compaction operations are not functionally required
kernel actions. Therefore making khugepaged tolerant to poisoned memory
will greatly improve user experience.

This patch series is for cases where khugepaged is the first guy
that detects the memory errors on the poisoned pages. IOW, the pages
are not known to have memory errors when khugepaged collapsing gets to
them. In our observation, this happens frequently when the huge page
ratio of the system is relatively low, which is fairly common in
virtual machines running on cloud.

Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages.
The high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.

We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing
a group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These operations
need to be undone in order to ensure these pages are not changed/lost from
the perspective of other threads (both user and kernel space). As for
file backed memory pages, there already exists a rollback case. This
patch just extends it so that khugepaged also correctly rolls back when
it fails to copy poisoned 4K pages.

Jiaqi Yan (2):
  mm: khugepaged: recover from poisoned anonymous memory
  mm: khugepaged: recover from poisoned file-backed memory

 include/linux/highmem.h |  18 ++++
 mm/khugepaged.c         | 215 ++++++++++++++++++++++++++++------------
 2 files changed, 171 insertions(+), 62 deletions(-)

-- 
2.35.1.1094.g7c7d902a7c-goog

[RFC v2 1/2] mm: khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan]

Make __collapse_huge_page_copy return whether
collapsing/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. Roll back the page table and page states
in the 2nd loop when copy failed:
1) re-establish the PTEs-to-PMD connection.
2) release pages back to their LRU list.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 include/linux/highmem.h |  18 ++++++
 mm/khugepaged.c         | 138 ++++++++++++++++++++++++++++++----------
 2 files changed, 123 insertions(+), 33 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 39bb9b47fa9cd..5d4b2af113496 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -296,6 +296,24 @@ static inline void copy_highpage(struct page *to, struct page *from)
 	kunmap_local(vfrom);
 }
 
+/*
+ * Machine check exception handled version of copy_highpage.
+ * Return true if copying page content failed; otherwise false.
+ */
+static inline bool copy_highpage_mc(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);
+	kunmap_local(vto);
+	kunmap_local(vfrom);
+
+	return ret > 0;
+}
+
 #endif
 
 static inline void memcpy_page(struct page *dst_page, size_t dst_off,
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 131492fd1148b..eecaeaf6eaebe 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -52,6 +52,7 @@ enum scan_result {
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_COPY_MC,
 };
 
 #define CREATE_TRACE_POINTS
@@ -739,44 +740,98 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	return 0;
 }
 
-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. Cleanup the normal pages if copy succeeds;
+ * otherwise restore the original pmd page table. Returns true if copy
+ * succeeds, otherwise returns false.
+ *
+ * @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 PTEs' PMD
+ * @address: starting address to copy
+ * @pte_ptl: lock on normal pages' PTEs
+ * @compound_pagelist: list that stores compound pages
+ */
+static bool __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;
+	bool copy_succeeded = true;
 
-		if (pte_none(pteval) || 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);
-			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
-			if (is_zero_pfn(pte_pfn(pteval))) {
-				/*
-				 * ptl mostly unnecessary.
-				 */
-				spin_lock(ptl);
-				ptep_clear(vma->vm_mm, address, _pte);
-				spin_unlock(ptl);
+		else {
+			src_page = pte_page(pteval);
+			if (copy_highpage_mc(page, src_page)) {
+				copy_succeeded = false;
+				break;
+			}
+		}
+	}
+
+	if (!copy_succeeded) {
+		/*
+		 * Copying failed, re-establish the regular PMD that
+		 * points to regular page table. Since PTEs are still
+		 * isolated and locked, acquiring anon_vma_lock is unnecessary.
+		 */
+		pmd_ptl = pmd_lock(vma->vm_mm, pmd);
+		pmd_populate(vma->vm_mm, pmd, pmd_pgtable(rollback));
+		spin_unlock(pmd_ptl);
+	}
+
+	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))) {
+			if (copy_succeeded) {
+				add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
+				if (is_zero_pfn(pte_pfn(pteval))) {
+					/*
+					 * ptl mostly unnecessary.
+					 */
+					spin_lock(pte_ptl);
+					pte_clear(vma->vm_mm, _address, _pte);
+					spin_unlock(pte_ptl);
+				}
 			}
 		} 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, false);
-			spin_unlock(ptl);
-			free_page_and_swap_cache(src_page);
+
+			if (copy_succeeded) {
+				/*
+				 * ptl mostly unnecessary, but preempt has to
+				 * be disabled to update the per-cpu stats
+				 * inside page_remove_rmap().
+				 */
+				spin_lock(pte_ptl);
+				pte_clear(vma->vm_mm, _address, _pte);
+				page_remove_rmap(src_page, false);
+				spin_unlock(pte_ptl);
+				free_page_and_swap_cache(src_page);
+			}
 		}
 	}
 
@@ -784,6 +839,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
 		list_del(&src_page->lru);
 		release_pte_page(src_page);
 	}
+
+	return copy_succeeded;
 }
 
 static void khugepaged_alloc_sleep(void)
@@ -1066,6 +1123,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
 	gfp_t gfp;
+	bool copied = false;
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
@@ -1177,9 +1235,13 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 */
 	anon_vma_unlock_write(vma->anon_vma);
 
-	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
-			&compound_pagelist);
+	copied = __collapse_huge_page_copy(pte, new_page, pmd, _pmd,
+			vma, address, pte_ptl, &compound_pagelist);
 	pte_unmap(pte);
+	if (!copied) {
+		result = SCAN_COPY_MC;
+		goto out_up_write;
+	}
 	/*
 	 * spin_lock() below is not the equivalent of smp_wmb(), but
 	 * the smp_wmb() inside __SetPageUptodate() can be reused to
@@ -1364,9 +1426,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 	pte_unmap_unlock(pte, ptl);
 	if (ret) {
 		node = khugepaged_find_target_node();
-		/* collapse_huge_page will return with the mmap_lock released */
-		collapse_huge_page(mm, address, hpage, node,
-				referenced, unmapped);
+		/*
+		 * collapse_huge_page will return with the mmap_r+w_lock released.
+		 * It is uncertain if *hpage is NULL or not when collapse_huge_page
+		 * returns, so keep ret=1 to jump to breakouterloop_mmap_lock
+		 * in khugepaged_scan_mm_slot, then *hpage will be freed
+		 * if collapse failed.
+		 */
+		collapse_huge_page(mm, address, hpage, node, referenced, unmapped);
 	}
 out:
 	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
@@ -2168,6 +2235,11 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
 				khugepaged_scan_file(mm, file, pgoff, hpage);
 				fput(file);
 			} else {
+				/*
+				 * mmap_read_lock is
+				 * 1) released if both scan and collapse succeeded;
+				 * 2) still held if either scan or collapse failed.
+				 */
 				ret = khugepaged_scan_pmd(mm, vma,
 						khugepaged_scan.address,
 						hpage);
-- 
2.35.1.1094.g7c7d902a7c-goog

[RFC v2 2/2] mm: khugepaged: recover from poisoned file-backed memory

[Jiaqi Yan]

Make collapse_file roll back when copying pages failed.
More concretely:
* extract copy operations into a separate loop
* postpone the updates for nr_none until both scan and copy succeeded
* postpone joining small xarray entries until both scan and copy
  succeeded
* postpone the update operations to NR_XXX_THPS
* for non-SHMEM file, roll back filemap_nr_thps_inc if scan
  succeeded but copy failed

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 mm/khugepaged.c | 77 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 48 insertions(+), 29 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index eecaeaf6eaebe..0c06758395931 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1710,7 +1710,7 @@ static void collapse_file(struct mm_struct *mm,
 {
 	struct address_space *mapping = file->f_mapping;
 	gfp_t gfp;
-	struct page *new_page;
+	struct page *new_page, *page, *tmp;
 	pgoff_t index, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
@@ -1766,7 +1766,7 @@ static void collapse_file(struct mm_struct *mm,
 
 	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-		struct page *page = xas_next(&xas);
+		page = xas_next(&xas);
 
 		VM_BUG_ON(index != xas.xa_index);
 		if (is_shmem) {
@@ -1938,10 +1938,7 @@ static void collapse_file(struct mm_struct *mm,
 	}
 	nr = thp_nr_pages(new_page);
 
-	if (is_shmem)
-		__mod_lruvec_page_state(new_page, NR_SHMEM_THPS, nr);
-	else {
-		__mod_lruvec_page_state(new_page, NR_FILE_THPS, nr);
+	if (!is_shmem) {
 		filemap_nr_thps_inc(mapping);
 		/*
 		 * Paired with smp_mb() in do_dentry_open() to ensure
@@ -1952,40 +1949,44 @@ static void collapse_file(struct mm_struct *mm,
 		smp_mb();
 		if (inode_is_open_for_write(mapping->host)) {
 			result = SCAN_FAIL;
-			__mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr);
 			filemap_nr_thps_dec(mapping);
 			goto xa_locked;
 		}
 	}
 
-	if (nr_none) {
-		__mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
-		if (is_shmem)
-			__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
-	}
-
-	/* Join all the small entries into a single multi-index entry */
-	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
-	xas_store(&xas, new_page);
 xa_locked:
 	xas_unlock_irq(&xas);
 xa_unlocked:
 
 	if (result == SCAN_SUCCEED) {
-		struct page *page, *tmp;
-
 		/*
 		 * Replacing old pages with new one has succeeded, now we
-		 * need to copy the content and free the old pages.
+		 * attempt to copy the contents.
 		 */
 		index = start;
-		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
+		list_for_each_entry(page, &pagelist, lru) {
 			while (index < page->index) {
 				clear_highpage(new_page + (index % HPAGE_PMD_NR));
 				index++;
 			}
-			copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
-					page);
+			if (copy_highpage_mc(new_page + (page->index % HPAGE_PMD_NR), page)) {
+				result = SCAN_COPY_MC;
+				break;
+			}
+			index++;
+		}
+		while (result == SCAN_SUCCEED && index < end) {
+			clear_highpage(new_page + (page->index % HPAGE_PMD_NR));
+			index++;
+		}
+	}
+
+	if (result == SCAN_SUCCEED) {
+		/*
+		 * Copying old pages to huge one has succeeded, now we
+		 * need to free the old pages.
+		 */
+		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
 			list_del(&page->lru);
 			page->mapping = NULL;
 			page_ref_unfreeze(page, 1);
@@ -1993,12 +1994,23 @@ static void collapse_file(struct mm_struct *mm,
 			ClearPageUnevictable(page);
 			unlock_page(page);
 			put_page(page);
-			index++;
 		}
-		while (index < end) {
-			clear_highpage(new_page + (index % HPAGE_PMD_NR));
-			index++;
+
+		xas_lock_irq(&xas);
+		if (is_shmem)
+			__mod_lruvec_page_state(new_page, NR_SHMEM_THPS, nr);
+		else
+			__mod_lruvec_page_state(new_page, NR_FILE_THPS, nr);
+
+		if (nr_none) {
+			__mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
+			if (is_shmem)
+				__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
 		}
+		/* Join all the small entries into a single multi-index entry. */
+		xas_set_order(&xas, start, HPAGE_PMD_ORDER);
+		xas_store(&xas, new_page);
+		xas_unlock_irq(&xas);
 
 		SetPageUptodate(new_page);
 		page_ref_add(new_page, HPAGE_PMD_NR - 1);
@@ -2014,9 +2026,9 @@ static void collapse_file(struct mm_struct *mm,
 
 		khugepaged_pages_collapsed++;
 	} else {
-		struct page *page;
-
-		/* Something went wrong: roll back page cache changes */
+		/*
+		 * Something went wrong: roll back page cache changes
+		 */
 		xas_lock_irq(&xas);
 		mapping->nrpages -= nr_none;
 
@@ -2049,6 +2061,13 @@ static void collapse_file(struct mm_struct *mm,
 			xas_lock_irq(&xas);
 		}
 		VM_BUG_ON(nr_none);
+		/*
+		 * Undo the updates of filemap_nr_thps_inc for non-SHMEM file only.
+		 * This undo is not needed unless failure is due to SCAN_COPY_MC.
+		 */
+		if (!is_shmem && result == SCAN_COPY_MC)
+			filemap_nr_thps_dec(mapping);
+
 		xas_unlock_irq(&xas);
 
 		new_page->mapping = NULL;
-- 
2.35.1.1094.g7c7d902a7c-goog

Re: [RFC v2 1/2] mm: khugepaged: recover from poisoned anonymous memory

[Jiaqi Yan]

Replacement patch to fix build error on architectures that define
__HAVE_ARCH_COPY_HIGHPAGE, i.e. move copy_highpage_mc out
of the "#ifndef __HAVE_ARCH_COPY_HIGHPAGE" block.

From b1225b762c075652675c61088dfa0b4d4b59ab90 Mon Sep 17 00:00:00 2001
From: Jiaqi Yan <jiaqiyan@google.com>
Date: Wed, 16 Feb 2022 01:04:25 +0000
Subject: [RFC v2 1/2] mm: khugepaged: recover from poisoned anonymous memory

Make __collapse_huge_page_copy return whether
collapsing/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. Roll back the page table and page states
in the 2nd loop when copying failed:
1) re-establish the PTEs-to-PMD connection.
2) release pages back to their LRU list.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 include/linux/highmem.h |  19 ++++++
 mm/khugepaged.c         | 138 ++++++++++++++++++++++++++++++----------
 2 files changed, 124 insertions(+), 33 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 39bb9b47fa9cd..0ccb1e92c4b06 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -298,6 +298,25 @@ static inline void copy_highpage(struct page *to,
struct page *from)

 #endif

+/*
+ * Machine check exception handled version of copy_highpage.
+ * Return true if copying page content failed; otherwise false.
+ * Note handling #MC requires arch opt-in.
+ */
+static inline bool copy_highpage_mc(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);
+ kunmap_local(vto);
+ kunmap_local(vfrom);
+
+ return ret > 0;
+}
+
 static inline void memcpy_page(struct page *dst_page, size_t dst_off,
         struct page *src_page, size_t src_off,
         size_t len)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 131492fd1148b..8e69a0640e551 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -52,6 +52,7 @@ enum scan_result {
  SCAN_CGROUP_CHARGE_FAIL,
  SCAN_TRUNCATED,
  SCAN_PAGE_HAS_PRIVATE,
+ SCAN_COPY_MC,
 };

 #define CREATE_TRACE_POINTS
@@ -739,44 +740,98 @@ static int __collapse_huge_page_isolate(struct
vm_area_struct *vma,
  return 0;
 }

-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. Cleanup the normal pages if copying succeeds;
+ * otherwise restore the original pmd page table. Returns true if copying
+ * succeeds, otherwise returns false.
+ *
+ * @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 PTEs' PMD
+ * @address: starting address to copy
+ * @pte_ptl: lock on normal pages' PTEs
+ * @compound_pagelist: list that stores compound pages
+ */
+static bool __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;
+ bool copy_succeeded = true;

- if (pte_none(pteval) || 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);
- add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
- if (is_zero_pfn(pte_pfn(pteval))) {
- /*
- * ptl mostly unnecessary.
- */
- spin_lock(ptl);
- ptep_clear(vma->vm_mm, address, _pte);
- spin_unlock(ptl);
+ else {
+ src_page = pte_page(pteval);
+ if (copy_highpage_mc(page, src_page)) {
+ copy_succeeded = false;
+ break;
+ }
+ }
+ }
+
+ if (!copy_succeeded) {
+ /*
+ * Copying failed, re-establish the regular PMD that
+ * points to regular page table. Since PTEs are still
+ * isolated and locked, acquiring anon_vma_lock is unnecessary.
+ */
+ pmd_ptl = pmd_lock(vma->vm_mm, pmd);
+ pmd_populate(vma->vm_mm, pmd, pmd_pgtable(rollback));
+ spin_unlock(pmd_ptl);
+ }
+
+ 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))) {
+ if (copy_succeeded) {
+ add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
+ if (is_zero_pfn(pte_pfn(pteval))) {
+ /*
+ * ptl mostly unnecessary.
+ */
+ spin_lock(pte_ptl);
+ pte_clear(vma->vm_mm, _address, _pte);
+ spin_unlock(pte_ptl);
+ }
  }
  } 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, false);
- spin_unlock(ptl);
- free_page_and_swap_cache(src_page);
+
+ if (copy_succeeded) {
+ /*
+ * ptl mostly unnecessary, but preempt has to
+ * be disabled to update the per-cpu stats
+ * inside page_remove_rmap().
+ */
+ spin_lock(pte_ptl);
+ pte_clear(vma->vm_mm, _address, _pte);
+ page_remove_rmap(src_page, false);
+ spin_unlock(pte_ptl);
+ free_page_and_swap_cache(src_page);
+ }
  }
  }

@@ -784,6 +839,8 @@ static void __collapse_huge_page_copy(pte_t *pte,
struct page *page,
  list_del(&src_page->lru);
  release_pte_page(src_page);
  }
+
+ return copy_succeeded;
 }

 static void khugepaged_alloc_sleep(void)
@@ -1066,6 +1123,7 @@ static void collapse_huge_page(struct mm_struct *mm,
  struct vm_area_struct *vma;
  struct mmu_notifier_range range;
  gfp_t gfp;
+ bool copied = false;

  VM_BUG_ON(address & ~HPAGE_PMD_MASK);

@@ -1177,9 +1235,13 @@ static void collapse_huge_page(struct mm_struct *mm,
  */
  anon_vma_unlock_write(vma->anon_vma);

- __collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
- &compound_pagelist);
+ copied = __collapse_huge_page_copy(pte, new_page, pmd, _pmd,
+ vma, address, pte_ptl, &compound_pagelist);
  pte_unmap(pte);
+ if (!copied) {
+ result = SCAN_COPY_MC;
+ goto out_up_write;
+ }
  /*
  * spin_lock() below is not the equivalent of smp_wmb(), but
  * the smp_wmb() inside __SetPageUptodate() can be reused to
@@ -1364,9 +1426,14 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
  pte_unmap_unlock(pte, ptl);
  if (ret) {
  node = khugepaged_find_target_node();
- /* collapse_huge_page will return with the mmap_lock released */
- collapse_huge_page(mm, address, hpage, node,
- referenced, unmapped);
+ /*
+ * collapse_huge_page will return with the mmap_r+w_lock released.
+ * It is uncertain if *hpage is NULL or not when collapse_huge_page
+ * returns, so keep ret=1 to jump to breakouterloop_mmap_lock
+ * in khugepaged_scan_mm_slot, then *hpage will be freed
+ * if collapse failed.
+ */
+ collapse_huge_page(mm, address, hpage, node, referenced, unmapped);
  }
 out:
  trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
@@ -2168,6 +2235,11 @@ static unsigned int
khugepaged_scan_mm_slot(unsigned int pages,
  khugepaged_scan_file(mm, file, pgoff, hpage);
  fput(file);
  } else {
+ /*
+ * mmap_read_lock is
+ * 1) released if both scan and collapse succeeded;
+ * 2) still held if either scan or collapse failed.
+ */
  ret = khugepaged_scan_pmd(mm, vma,
  khugepaged_scan.address,
  hpage);
-- 
2.35.1.1178.g4f1659d476-goog

Re: [RFC v2 1/2] mm: khugepaged: recover from poisoned anonymous memory

[kernel test robot]

Hi Jiaqi,

[FYI, it's a private test report for your RFC patch.]
[auto build test ERROR on v5.17]
[cannot apply to hnaz-mm/master linus/master v5.18-rc1 next-20220406]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/intel-lab-lkp/linux/commits/Jiaqi-Yan/Memory-poison-recovery-in-khugepaged-collapsing/20220406-045440
base:    f443e374ae131c168a065ea1748feac6b2e76613
config: arm64-randconfig-r036-20220406 (https://download.01.org/0day-ci/archive/20220407/202204070554.j055zHGv-lkp@intel.com/config)
compiler: clang version 15.0.0 (https://github.com/llvm/llvm-project c4a1b07d0979e7ff20d7d541af666d822d66b566)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install arm64 cross compiling tool for clang build
        # apt-get install binutils-aarch64-linux-gnu
        # https://github.com/intel-lab-lkp/linux/commit/e2c6963af941ef8a75c397d2c911113d59bfef75
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Jiaqi-Yan/Memory-poison-recovery-in-khugepaged-collapsing/20220406-045440
        git checkout e2c6963af941ef8a75c397d2c911113d59bfef75
        # save the config file to linux build tree
        mkdir build_dir
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm64 SHELL=/bin/bash

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

>> mm/khugepaged.c:785:8: error: implicit declaration of function 'copy_highpage_mc' [-Werror,-Wimplicit-function-declaration]
                           if (copy_highpage_mc(page, src_page)) {
                               ^
   mm/khugepaged.c:785:8: note: did you mean 'copy_highpage'?
   arch/arm64/include/asm/page.h:29:6: note: 'copy_highpage' declared here
   void copy_highpage(struct page *to, struct page *from);
        ^
   1 error generated.


vim +/copy_highpage_mc +785 mm/khugepaged.c

   742	
   743	/*
   744	 * __collapse_huge_page_copy - attempts to copy memory contents from normal
   745	 * pages to a hugepage. Cleanup the normal pages if copy succeeds;
   746	 * otherwise restore the original pmd page table. Returns true if copy
   747	 * succeeds, otherwise returns false.
   748	 *
   749	 * @pte: starting of the PTEs to copy from
   750	 * @page: the new hugepage to copy contents to
   751	 * @pmd: pointer to the new hugepage's PMD
   752	 * @rollback: the original normal PTEs' PMD
   753	 * @address: starting address to copy
   754	 * @pte_ptl: lock on normal pages' PTEs
   755	 * @compound_pagelist: list that stores compound pages
   756	 */
   757	static bool __collapse_huge_page_copy(pte_t *pte,
   758					struct page *page,
   759					pmd_t *pmd,
   760					pmd_t rollback,
   761					struct vm_area_struct *vma,
   762					unsigned long address,
   763					spinlock_t *pte_ptl,
   764					struct list_head *compound_pagelist)
   765	{
   766		struct page *src_page, *tmp;
   767		pte_t *_pte;
   768		pte_t pteval;
   769		unsigned long _address;
   770		spinlock_t *pmd_ptl;
   771		bool copy_succeeded = true;
   772	
   773		/*
   774		 * Copying pages' contents is subject to memory poison at any iteration.
   775		 */
   776		for (_pte = pte, _address = address;
   777				_pte < pte + HPAGE_PMD_NR;
   778				_pte++, page++, _address += PAGE_SIZE) {
   779			pteval = *_pte;
   780	
   781			if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
   782				clear_user_highpage(page, address);
   783			else {
   784				src_page = pte_page(pteval);
 > 785				if (copy_highpage_mc(page, src_page)) {
   786					copy_succeeded = false;
   787					break;
   788				}
   789			}
   790		}
   791	
   792		if (!copy_succeeded) {
   793			/*
   794			 * Copying failed, re-establish the regular PMD that
   795			 * points to regular page table. Since PTEs are still
   796			 * isolated and locked, acquiring anon_vma_lock is unnecessary.
   797			 */
   798			pmd_ptl = pmd_lock(vma->vm_mm, pmd);
   799			pmd_populate(vma->vm_mm, pmd, pmd_pgtable(rollback));
   800			spin_unlock(pmd_ptl);
   801		}
   802	
   803		for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
   804				_pte++, _address += PAGE_SIZE) {
   805			pteval = *_pte;
   806			if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
   807				if (copy_succeeded) {
   808					add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
   809					if (is_zero_pfn(pte_pfn(pteval))) {
   810						/*
   811						 * ptl mostly unnecessary.
   812						 */
   813						spin_lock(pte_ptl);
   814						pte_clear(vma->vm_mm, _address, _pte);
   815						spin_unlock(pte_ptl);
   816					}
   817				}
   818			} else {
   819				src_page = pte_page(pteval);
   820				if (!PageCompound(src_page))
   821					release_pte_page(src_page);
   822	
   823				if (copy_succeeded) {
   824					/*
   825					 * ptl mostly unnecessary, but preempt has to
   826					 * be disabled to update the per-cpu stats
   827					 * inside page_remove_rmap().
   828					 */
   829					spin_lock(pte_ptl);
   830					pte_clear(vma->vm_mm, _address, _pte);
   831					page_remove_rmap(src_page, false);
   832					spin_unlock(pte_ptl);
   833					free_page_and_swap_cache(src_page);
   834				}
   835			}
   836		}
   837	
   838		list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
   839			list_del(&src_page->lru);
   840			release_pte_page(src_page);
   841		}
   842	
   843		return copy_succeeded;
   844	}
   845	

-- 
0-DAY CI Kernel Test Service
https://01.org/lkp

[RFC v2 0/2] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors.
As memory size and density increase, the chances of and number of
memory errors increase. The increasing size and density of server
RAM in the data center and cloud have shown increased uncorrectable
memory errors. There are already mechanisms in the kernel to recover
from uncorrectable memory errors. This series of patches provides
the recovery mechanism for the particular kernel agent khugepaged
when it collapses memory pages.

Impact
======
The main reason we chose to make khugepaged collapsing tolerant of
memory failures was its high possibility of accessing poisoned memory
while performing functionally optional compaction actions.
Standard applications typically don't have strict requirements on
the size of its pages. So they are given 4K pages by the kernel.
The kernel is able to improve application performance by either

  1) giving applications 2M pages to begin with, or
  2) collapsing 4K pages into 2M pages when possible.

This collapsing operation is done by khugepaged, a kernel agent that
is constantly scanning memory. When collapsing 4K pages into a 2M page,
it must copy the data from the 4K pages into a physically contiguous
2M page. Therefore, as long as there exists one poisoned cache line in
collapsible 4K pages, khugepaged will eventually access it. The current
impact to users is a machine check exception triggered kernel panic.
However, khugepaged’s compaction operations are not functionally required
kernel actions. Therefore making khugepaged tolerant to poisoned memory
will greatly improve user experience.

This patch series is for cases where khugepaged is the first guy
that detects the memory errors on the poisoned pages. IOW, the pages
are not known to have memory errors when khugepaged collapsing gets to
them. In our observation, this happens frequently when the huge page
ratio of the system is relatively low, which is fairly common in
virtual machines running on cloud.

Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages.
The high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.

We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing
a group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These operations
need to be undone in order to ensure these pages are not changed/lost from
the perspective of other threads (both user and kernel space). As for
file backed memory pages, there already exists a rollback case. This
patch just extends it so that khugepaged also correctly rolls back when
it fails to copy poisoned 4K pages.

Jiaqi Yan (2):
  mm: khugepaged: recover from poisoned anonymous memory
  mm: khugepaged: recover from poisoned file-backed memory

 include/linux/highmem.h |  19 ++++
 mm/khugepaged.c         | 215 ++++++++++++++++++++++++++++------------
 2 files changed, 172 insertions(+), 62 deletions(-)

-- 
2.35.1.1178.g4f1659d476-goog

[RFC v2 0/2] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors.
As memory size and density increase, the chances of and number of
memory errors increase. The increasing size and density of server
RAM in the data center and cloud have shown increased uncorrectable
memory errors. There are already mechanisms in the kernel to recover
from uncorrectable memory errors. This series of patches provides
the recovery mechanism for the particular kernel agent khugepaged
when it collapses memory pages.

Impact
======
The main reason we chose to make khugepaged collapsing tolerant of
memory failures was its high possibility of accessing poisoned memory
while performing functionally optional compaction actions.
Standard applications typically don't have strict requirements on
the size of its pages. So they are given 4K pages by the kernel.
The kernel is able to improve application performance by either

  1) giving applications 2M pages to begin with, or
  2) collapsing 4K pages into 2M pages when possible.

This collapsing operation is done by khugepaged, a kernel agent that
is constantly scanning memory. When collapsing 4K pages into a 2M page,
it must copy the data from the 4K pages into a physically contiguous
2M page. Therefore, as long as there exists one poisoned cache line in
collapsible 4K pages, khugepaged will eventually access it. The current
impact to users is a machine check exception triggered kernel panic.
However, khugepaged’s compaction operations are not functionally required
kernel actions. Therefore making khugepaged tolerant to poisoned memory
will greatly improve user experience.

This patch series is for cases where khugepaged is the first guy
that detects the memory errors on the poisoned pages. IOW, the pages
are not known to have memory errors when khugepaged collapsing gets to
them. In our observation, this happens frequently when the huge page
ratio of the system is relatively low, which is fairly common in
virtual machines running on cloud.

Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages.
The high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.

We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing
a group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These operations
need to be undone in order to ensure these pages are not changed/lost from
the perspective of other threads (both user and kernel space). As for
file backed memory pages, there already exists a rollback case. This
patch just extends it so that khugepaged also correctly rolls back when
it fails to copy poisoned 4K pages.

Jiaqi Yan (2):
  mm: khugepaged: recover from poisoned anonymous memory
  mm: khugepaged: recover from poisoned file-backed memory

 include/linux/highmem.h |  19 ++++
 mm/khugepaged.c         | 215 ++++++++++++++++++++++++++++------------
 2 files changed, 172 insertions(+), 62 deletions(-)

-- 
2.35.1.1178.g4f1659d476-goog