[PATCH v9 0/2] Memory poison recovery in khugepaged collapsing

[PATCH v9 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.

Changelog
=========

v9 changes
- Incorporate feedback from Andrew Morton <akpm@linux-foundation.org>
- Move copy_mc_highpage into khugepage.c as a static out-of-line
  function copy_mc_page.

v8 changes
- Incorporate feedbacks from Tony Luck <tony.luck@intel.com>
- Rename copy_highpage_mc to copy_mc_highpage.
- Update copy_mc_highpage with kmsan changes.
- Code style changes:
  1) copy_mc_highpage returns int as "copy" is an action and is consistent
     with copy_mc_user_highpage.
  2) __collapse_huge_page_copy returns scan_result(int) and is consistent
     with __collapse_huge_page_isolate/swapin.
  3) variables are declared in separate lines in collapse_file.

v7 changes
- Fix a bug "KASAN: stack-out-of-bounds Read in collapse_file". After copying
  all pages into the huge page, clear_highpage should use `index` instead of
  `page->index`.

v6 changes
- Address comments from Kirill Shutemov <kirill@shutemov.name>
- Rewrite __collapse_huge_page_copy to make rollback operations more
  clear to its reader.
- Add detailed test steps in each commit message.

v5 changes
- Rebase patches to mm-unstable at
  commit ffb39098bf87 ("Merge tag 'linux-kselftest-kunit-6.1-rc1' of
  git://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest").
- Resolves conflicts with:
  commit 2f55f070e5b8 ("mm/khugepaged: minor cleanup for collapse_file")
  commit 1baec203b77c ("mm/khugepaged: try to free transhuge swapcache
  when possible")

v4 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Remove tracepoint for __collapse_huge_page_copy, just keep SCAN_COPY_MC
  and let trace_mm_collapse_huge_page it
- Remove unnecessary comments

v3 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Add tracepoint for __collapse_huge_page_copy
- Restore PMD in collapse_huge_page
- Correct comment about mmap_read_lock

v2 changes
- Incorporate feedbacks from Yang Shi <shy828301@gmail.com>
- Only keep copy_highpage_mc
- Adding new scan_result SCAN_COPY_MC
- Defer NR_FILE_THPS update until copying succeeded

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

 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 257 +++++++++++++++++++++--------
 2 files changed, 187 insertions(+), 73 deletions(-)

-- 
2.39.0.rc0.267.gcb52ba06e7-goog

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

[Jiaqi Yan]

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

[PATCH v9 2/2] mm/khugepaged: recover from poisoned file-backed memory

[Jiaqi Yan]

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

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

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 48 insertions(+), 30 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 0f1b9e05e17ec..ce68e76c1f0f2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1839,6 +1839,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 {
 	struct address_space *mapping = file->f_mapping;
 	struct page *hpage;
+	struct page *page;
+	struct page *tmp;
+	struct folio *folio;
 	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
@@ -1883,8 +1886,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
 	xas_set(&xas, start);
 	for (index = start; index < end; index++) {
-		struct page *page = xas_next(&xas);
-		struct folio *folio;
+		page = xas_next(&xas);
 
 		VM_BUG_ON(index != xas.xa_index);
 		if (is_shmem) {
@@ -2066,10 +2068,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	}
 	nr = thp_nr_pages(hpage);
 
-	if (is_shmem)
-		__mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
-	else {
-		__mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
+	if (!is_shmem) {
 		filemap_nr_thps_inc(mapping);
 		/*
 		 * Paired with smp_mb() in do_dentry_open() to ensure
@@ -2080,21 +2079,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		smp_mb();
 		if (inode_is_open_for_write(mapping->host)) {
 			result = SCAN_FAIL;
-			__mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
 			filemap_nr_thps_dec(mapping);
 			goto xa_locked;
 		}
 	}
-
-	if (nr_none) {
-		__mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
-		/* nr_none is always 0 for non-shmem. */
-		__mod_lruvec_page_state(hpage, 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, hpage);
 xa_locked:
 	xas_unlock_irq(&xas);
 xa_unlocked:
@@ -2107,21 +2095,35 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	try_to_unmap_flush();
 
 	if (result == SCAN_SUCCEED) {
-		struct page *page, *tmp;
-		struct folio *folio;
-
 		/*
 		 * 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(hpage + (index % HPAGE_PMD_NR));
 				index++;
 			}
-			copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
-				      page);
+			if (copy_mc_page(hpage + (page->index % HPAGE_PMD_NR),
+					 page) > 0) {
+				result = SCAN_COPY_MC;
+				break;
+			}
+			index++;
+		}
+		while (result == SCAN_SUCCEED && index < end) {
+			clear_highpage(hpage + (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);
@@ -2129,12 +2131,23 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			ClearPageUnevictable(page);
 			unlock_page(page);
 			put_page(page);
-			index++;
 		}
-		while (index < end) {
-			clear_highpage(hpage + (index % HPAGE_PMD_NR));
-			index++;
+
+		xas_lock_irq(&xas);
+		if (is_shmem)
+			__mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
+		else
+			__mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
+
+		if (nr_none) {
+			__mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
+			/* nr_none is always 0 for non-shmem. */
+			__mod_lruvec_page_state(hpage, 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, hpage);
+		xas_unlock_irq(&xas);
 
 		folio = page_folio(hpage);
 		folio_mark_uptodate(folio);
@@ -2152,8 +2165,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 		unlock_page(hpage);
 		hpage = NULL;
 	} else {
-		struct page *page;
-
 		/* Something went wrong: roll back page cache changes */
 		xas_lock_irq(&xas);
 		if (nr_none) {
@@ -2187,6 +2198,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			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);
 
 		hpage->mapping = NULL;
-- 
2.39.0.rc0.267.gcb52ba06e7-goog

Re: [PATCH v9 0/2] Memory poison recovery in khugepaged collapsing

[Andrew Morton]

On Mon,  5 Dec 2022 15:40:57 -0800 Jiaqi Yan <jiaqiyan@google.com> wrote:

> 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.

Could we please have some reviewer input on this series?

Thanks.

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

[kirill.shutemov]

On Mon, Dec 05, 2022 at 03:40:58PM -0800, Jiaqi Yan wrote:
> 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;
> +}


It is very similar to copy_mc_user_highpage(), but uses
kmsan_copy_page_meta() instead of kmsan_unpoison_memory().

Could you explain the difference? I don't quite get it.

> +
>  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,

I think 'orig_pmd' is a better name.

> +				     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);

Indentation levels get out of control. Maybe some code restructuring is
required?

>  			}
> -		} 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
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH v9 2/2] mm/khugepaged: recover from poisoned file-backed memory

[kirill.shutemov]

On Mon, Dec 05, 2022 at 03:40:59PM -0800, Jiaqi Yan wrote:
> Make collapse_file roll back when copying pages failed. More concretely:
> - extract copying operations into a separate loop
> - postpone the updates for nr_none until both scanning and copying
>   succeeded
> - postpone joining small xarray entries until both scanning and copying
>   succeeded
> - postpone the update operations to NR_XXX_THPS until both scanning and
>   copying succeeded
> - for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
>   copying failed
> 
> Tested manually:
> 0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
> 1. Start a two-thread application. Each thread allocates a chunk of
>    non-huge memory buffer from /mnt/ramdisk.
> 2. Pick 4 random buffer address (2 in each thread) and inject
>    uncorrectable memory errors at physical addresses.
> 3. Signal both threads to make their memory buffer collapsible, i.e.
>    calling madvise(MADV_HUGEPAGE).
> 4. Wait and then check kernel log: khugepaged is able to recover from
>    poisoned pages by skipping them.
> 5. Signal both threads to inspect their buffer contents and make sure no
>    data corruption.
> 
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

Okay, looks sane.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

Re: [PATCH v9 2/2] mm/khugepaged: recover from poisoned file-backed memory

[Jiaqi Yan]

On Thu, Jan 19, 2023 at 7:10 AM <kirill.shutemov@linux.intel.com> wrote:
>
> On Mon, Dec 05, 2022 at 03:40:59PM -0800, Jiaqi Yan wrote:
> > Make collapse_file roll back when copying pages failed. More concretely:
> > - extract copying operations into a separate loop
> > - postpone the updates for nr_none until both scanning and copying
> >   succeeded
> > - postpone joining small xarray entries until both scanning and copying
> >   succeeded
> > - postpone the update operations to NR_XXX_THPS until both scanning and
> >   copying succeeded
> > - for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
> >   copying failed
> >
> > Tested manually:
> > 0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
> > 1. Start a two-thread application. Each thread allocates a chunk of
> >    non-huge memory buffer from /mnt/ramdisk.
> > 2. Pick 4 random buffer address (2 in each thread) and inject
> >    uncorrectable memory errors at physical addresses.
> > 3. Signal both threads to make their memory buffer collapsible, i.e.
> >    calling madvise(MADV_HUGEPAGE).
> > 4. Wait and then check kernel log: khugepaged is able to recover from
> >    poisoned pages by skipping them.
> > 5. Signal both threads to inspect their buffer contents and make sure no
> >    data corruption.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
>
> Okay, looks sane.

Thanks for your review, :).

>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

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

[Jiaqi Yan]

On Thu, Jan 19, 2023 at 7:03 AM <kirill.shutemov@linux.intel.com> wrote:
>
> On Mon, Dec 05, 2022 at 03:40:58PM -0800, Jiaqi Yan wrote:
> > 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;
> > +}
>
>
> It is very similar to copy_mc_user_highpage(), but uses
> kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
>
> Could you explain the difference? I don't quite get it.

copy_mc_page is actually the MC version of copy_highpage, which uses
kmsan_copy_page_meta instead of kmsan_unpoison_memory.

My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
When there is no metadata (kmsan_shadow or kmsan_origin), both
kmsan_copy_page_meta and kmsan_unpoison_memory just do
kmsan_internal_unpoison_memory to mark the memory range as
initialized; when there is metadata in src page, kmsan_copy_page_meta
will copy whatever metadata in src to dst. So I think
kmsan_copy_page_meta is the right thing to do.

>
> > +
> >  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,
>
> I think 'orig_pmd' is a better name.

Will be renamed to orig_pmd in the next version v10.

>
> > +                                  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);
>
> Indentation levels get out of control. Maybe some code restructuring is
> required?

v10 will change to something like this to reduce 1 level of indentation:

    if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
        continue;
    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
> >
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

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

[kirill.shutemov]

On Fri, Jan 20, 2023 at 07:56:15AM -0800, Jiaqi Yan wrote:
> On Thu, Jan 19, 2023 at 7:03 AM <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Mon, Dec 05, 2022 at 03:40:58PM -0800, Jiaqi Yan wrote:
> > > 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;
> > > +}
> >
> >
> > It is very similar to copy_mc_user_highpage(), but uses
> > kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
> >
> > Could you explain the difference? I don't quite get it.
> 
> copy_mc_page is actually the MC version of copy_highpage, which uses
> kmsan_copy_page_meta instead of kmsan_unpoison_memory.
> 
> My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
> When there is no metadata (kmsan_shadow or kmsan_origin), both
> kmsan_copy_page_meta and kmsan_unpoison_memory just do
> kmsan_internal_unpoison_memory to mark the memory range as
> initialized; when there is metadata in src page, kmsan_copy_page_meta
> will copy whatever metadata in src to dst. So I think
> kmsan_copy_page_meta is the right thing to do.

Should we fix copy_mc_user_highpage() then?

> > Indentation levels get out of control. Maybe some code restructuring is
> > required?
> 
> v10 will change to something like this to reduce 1 level of indentation:
> 
>     if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
>         continue;
>     src_page = pte_page(pteval);
>     if (!PageCompound(src_page))
>         release_pte_page(src_page);

I hoped for deeper rework. Maybe split the function into several functions
and make overall structure more readable?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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

[Jiaqi Yan]

On Mon, Jan 23, 2023 at 4:34 PM <kirill.shutemov@linux.intel.com> wrote:
>
> On Fri, Jan 20, 2023 at 07:56:15AM -0800, Jiaqi Yan wrote:
> > On Thu, Jan 19, 2023 at 7:03 AM <kirill.shutemov@linux.intel.com> wrote:
> > >
> > > On Mon, Dec 05, 2022 at 03:40:58PM -0800, Jiaqi Yan wrote:
> > > > 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;
> > > > +}
> > >
> > >
> > > It is very similar to copy_mc_user_highpage(), but uses
> > > kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
> > >
> > > Could you explain the difference? I don't quite get it.
> >
> > copy_mc_page is actually the MC version of copy_highpage, which uses
> > kmsan_copy_page_meta instead of kmsan_unpoison_memory.
> >
> > My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
> > When there is no metadata (kmsan_shadow or kmsan_origin), both
> > kmsan_copy_page_meta and kmsan_unpoison_memory just do
> > kmsan_internal_unpoison_memory to mark the memory range as
> > initialized; when there is metadata in src page, kmsan_copy_page_meta
> > will copy whatever metadata in src to dst. So I think
> > kmsan_copy_page_meta is the right thing to do.
>
> Should we fix copy_mc_user_highpage() then?

I think it depends on what copy_user_highpage() (the original of
copy_mc_user_highpage) is used for. copy_mc_user_highpage is currently
only used by __wp_page_copy_user, is it possible that here we don't
want to (or don't need to) copy page metadata for userspace pages?

>
> > > Indentation levels get out of control. Maybe some code restructuring is
> > > required?
> >
> > v10 will change to something like this to reduce 1 level of indentation:
> >
> >     if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
> >         continue;
> >     src_page = pte_page(pteval);
> >     if (!PageCompound(src_page))
> >         release_pte_page(src_page);
>
> I hoped for deeper rework. Maybe split the function into several functions
> and make overall structure more readable?

How about turning the 2nd loop into
__collapse_huge_page_copy_succeeded and
__collapse_huge_page_copy_failed, one for the case copy succeeded, and
one for failed? Like this:
  if (likely(result == SCAN_SUCCEED))
    __collapse_huge_page_copy_succeeded(...);
  else
    __collapse_huge_page_copy_failed(...);

My prototype shows it could reduce the level indents.

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

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

[kirill.shutemov]

On Tue, Jan 31, 2023 at 09:16:45PM -0800, Jiaqi Yan wrote:
> > > > > +/*
> > > > > + * 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;
> > > > > +}
> > > >
> > > >
> > > > It is very similar to copy_mc_user_highpage(), but uses
> > > > kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
> > > >
> > > > Could you explain the difference? I don't quite get it.
> > >
> > > copy_mc_page is actually the MC version of copy_highpage, which uses
> > > kmsan_copy_page_meta instead of kmsan_unpoison_memory.
> > >
> > > My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
> > > When there is no metadata (kmsan_shadow or kmsan_origin), both
> > > kmsan_copy_page_meta and kmsan_unpoison_memory just do
> > > kmsan_internal_unpoison_memory to mark the memory range as
> > > initialized; when there is metadata in src page, kmsan_copy_page_meta
> > > will copy whatever metadata in src to dst. So I think
> > > kmsan_copy_page_meta is the right thing to do.
> >
> > Should we fix copy_mc_user_highpage() then?
> 
> I think it depends on what copy_user_highpage() (the original of
> copy_mc_user_highpage) is used for. copy_mc_user_highpage is currently
> only used by __wp_page_copy_user, is it possible that here we don't
> want to (or don't need to) copy page metadata for userspace pages?

Tony, could chime in on this? Can we modify copy_mc_user_highpage() to
also use kmsan_copy_page_meta()? I don't really understand KMSAN here.
> 
> >
> > > > Indentation levels get out of control. Maybe some code restructuring is
> > > > required?
> > >
> > > v10 will change to something like this to reduce 1 level of indentation:
> > >
> > >     if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval)))
> > >         continue;
> > >     src_page = pte_page(pteval);
> > >     if (!PageCompound(src_page))
> > >         release_pte_page(src_page);
> >
> > I hoped for deeper rework. Maybe split the function into several functions
> > and make overall structure more readable?
> 
> How about turning the 2nd loop into
> __collapse_huge_page_copy_succeeded and
> __collapse_huge_page_copy_failed, one for the case copy succeeded, and
> one for failed? Like this:
>   if (likely(result == SCAN_SUCCEED))
>     __collapse_huge_page_copy_succeeded(...);
>   else
>     __collapse_huge_page_copy_failed(...);
> 
> My prototype shows it could reduce the level indents.

Give it a try and try to get into reader shoes. Get it easily digestible
for someone who reads the code for the first time.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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

[kirill]

On Thu, Feb 02, 2023 at 03:01:02AM +0300, kirill.shutemov@linux.intel.com wrote:
> On Tue, Jan 31, 2023 at 09:16:45PM -0800, Jiaqi Yan wrote:
> > > > > > +/*
> > > > > > + * 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;
> > > > > > +}
> > > > >
> > > > >
> > > > > It is very similar to copy_mc_user_highpage(), but uses
> > > > > kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
> > > > >
> > > > > Could you explain the difference? I don't quite get it.
> > > >
> > > > copy_mc_page is actually the MC version of copy_highpage, which uses
> > > > kmsan_copy_page_meta instead of kmsan_unpoison_memory.
> > > >
> > > > My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
> > > > When there is no metadata (kmsan_shadow or kmsan_origin), both
> > > > kmsan_copy_page_meta and kmsan_unpoison_memory just do
> > > > kmsan_internal_unpoison_memory to mark the memory range as
> > > > initialized; when there is metadata in src page, kmsan_copy_page_meta
> > > > will copy whatever metadata in src to dst. So I think
> > > > kmsan_copy_page_meta is the right thing to do.
> > >
> > > Should we fix copy_mc_user_highpage() then?
> > 
> > I think it depends on what copy_user_highpage() (the original of
> > copy_mc_user_highpage) is used for. copy_mc_user_highpage is currently
> > only used by __wp_page_copy_user, is it possible that here we don't
> > want to (or don't need to) copy page metadata for userspace pages?
> 
> Tony, could chime in on this? Can we modify copy_mc_user_highpage() to
> also use kmsan_copy_page_meta()? I don't really understand KMSAN here.

+KMSAN folks.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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

[Jiaqi Yan]

Pinging KMSAN experts, for the general guidance of
kmsan_copy_page_meta vs kmsan_unpoison_memory

On Wed, Feb 1, 2023 at 4:30 PM <kirill@shutemov.name> wrote:
>
> On Thu, Feb 02, 2023 at 03:01:02AM +0300, kirill.shutemov@linux.intel.com wrote:
> > On Tue, Jan 31, 2023 at 09:16:45PM -0800, Jiaqi Yan wrote:
> > > > > > > +/*
> > > > > > > + * 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;
> > > > > > > +}
> > > > > >
> > > > > >
> > > > > > It is very similar to copy_mc_user_highpage(), but uses
> > > > > > kmsan_copy_page_meta() instead of kmsan_unpoison_memory().
> > > > > >
> > > > > > Could you explain the difference? I don't quite get it.
> > > > >
> > > > > copy_mc_page is actually the MC version of copy_highpage, which uses
> > > > > kmsan_copy_page_meta instead of kmsan_unpoison_memory.
> > > > >
> > > > > My understanding is kmsan_copy_page_meta covers kmsan_unpoison_memory.
> > > > > When there is no metadata (kmsan_shadow or kmsan_origin), both
> > > > > kmsan_copy_page_meta and kmsan_unpoison_memory just do
> > > > > kmsan_internal_unpoison_memory to mark the memory range as
> > > > > initialized; when there is metadata in src page, kmsan_copy_page_meta
> > > > > will copy whatever metadata in src to dst. So I think
> > > > > kmsan_copy_page_meta is the right thing to do.
> > > >
> > > > Should we fix copy_mc_user_highpage() then?
> > >
> > > I think it depends on what copy_user_highpage() (the original of
> > > copy_mc_user_highpage) is used for. copy_mc_user_highpage is currently
> > > only used by __wp_page_copy_user, is it possible that here we don't
> > > want to (or don't need to) copy page metadata for userspace pages?
> >
> > Tony, could chime in on this? Can we modify copy_mc_user_highpage() to
> > also use kmsan_copy_page_meta()? I don't really understand KMSAN here.
>
> +KMSAN folks.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

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

[Alexander Potapenko]

On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> Pinging KMSAN experts, for the general guidance of
> kmsan_copy_page_meta vs kmsan_unpoison_memory

Oh, sorry, I've missed the previous email.

copy_mc_user_highpage() is expected to copy data from the user page,
for which no metadata is ever allocated.
Therefore we just initialize the destination shadow with zeros instead
of copying anything.

kmsan_copy_page_meta() is used when the metadata is copied between two
kernel pages, therefore it handles the cases when page->kmsan_shadow
is NULL for the source and destination pages.

It might be a good idea to use kmsan_copy_page_meta() in both cases,
but to do that I want to better understand what happens when
kmap_local_page(from) is called in copy_mc_user_highpage().
Where does the corresponding struct page come from?

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

[Jiaqi Yan]

On Wed, Feb 8, 2023 at 3:45 AM Alexander Potapenko <glider@google.com> wrote:
>
> On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> >
> > Pinging KMSAN experts, for the general guidance of
> > kmsan_copy_page_meta vs kmsan_unpoison_memory
>
> Oh, sorry, I've missed the previous email.

NP, thanks for jumping in :)

>
> copy_mc_user_highpage() is expected to copy data from the user page,
> for which no metadata is ever allocated.
> Therefore we just initialize the destination shadow with zeros instead
> of copying anything.
>
> kmsan_copy_page_meta() is used when the metadata is copied between two
> kernel pages, therefore it handles the cases when page->kmsan_shadow
> is NULL for the source and destination pages.
>
> It might be a good idea to use kmsan_copy_page_meta() in both cases,
> but to do that I want to better understand what happens when
> kmap_local_page(from) is called in copy_mc_user_highpage().
> Where does the corresponding struct page come from?

Kirill can correct me, but I think khugepaged always copies user pages
because it is trying to convert raw pages to THP for better userspace
application performance. Therefore khugepaged should only need
copy_mc_user_highpage(), for both file-backed and anonymous memory
pages.

However, copy_mc_user_highpage() needs both vaddr and vma, so it is a
little bit hard to use it in collapse_file (i.e. in the file-backed
case):
1. vma is not carried over to collapse_file from khugepaged_scan_mm_slot
2. collapse_file is not directly iterating with vaddrs of pages to be copied

(Although both vaddr and vma are unused auguments in
copy_mc_user_highpage, I think for cleanness, the caller e.g.
khugepaged should feed valid values).

So my patchset uses copy_mc_page(and kmsan_copy_page_meta) for both
file-backed and anon memory pages. I guess as long as
kmsan_copy_page_meta doesn't do anything unexpected for user pages (at
least from my reading), we are good?

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

[Jiaqi Yan]

Hi Kirill,

Given the discussion, are you ok with `copy_mc_page` introduced in
this patch? If so I am going to send out v10 that has a major refactor
on __collapse_huge_page_copy.

Thanks,
Jiaqi

On Wed, Feb 8, 2023 at 3:00 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> On Wed, Feb 8, 2023 at 3:45 AM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> > >
> > > Pinging KMSAN experts, for the general guidance of
> > > kmsan_copy_page_meta vs kmsan_unpoison_memory
> >
> > Oh, sorry, I've missed the previous email.
>
> NP, thanks for jumping in :)
>
> >
> > copy_mc_user_highpage() is expected to copy data from the user page,
> > for which no metadata is ever allocated.
> > Therefore we just initialize the destination shadow with zeros instead
> > of copying anything.
> >
> > kmsan_copy_page_meta() is used when the metadata is copied between two
> > kernel pages, therefore it handles the cases when page->kmsan_shadow
> > is NULL for the source and destination pages.
> >
> > It might be a good idea to use kmsan_copy_page_meta() in both cases,
> > but to do that I want to better understand what happens when
> > kmap_local_page(from) is called in copy_mc_user_highpage().
> > Where does the corresponding struct page come from?
>
> Kirill can correct me, but I think khugepaged always copies user pages
> because it is trying to convert raw pages to THP for better userspace
> application performance. Therefore khugepaged should only need
> copy_mc_user_highpage(), for both file-backed and anonymous memory
> pages.
>
> However, copy_mc_user_highpage() needs both vaddr and vma, so it is a
> little bit hard to use it in collapse_file (i.e. in the file-backed
> case):
> 1. vma is not carried over to collapse_file from khugepaged_scan_mm_slot
> 2. collapse_file is not directly iterating with vaddrs of pages to be copied
>
> (Although both vaddr and vma are unused auguments in
> copy_mc_user_highpage, I think for cleanness, the caller e.g.
> khugepaged should feed valid values).
>
> So my patchset uses copy_mc_page(and kmsan_copy_page_meta) for both
> file-backed and anon memory pages. I guess as long as
> kmsan_copy_page_meta doesn't do anything unexpected for user pages (at
> least from my reading), we are good?

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

[kirill]

On Wed, Feb 08, 2023 at 03:00:59PM -0800, Jiaqi Yan wrote:
> On Wed, Feb 8, 2023 at 3:45 AM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> > >
> > > Pinging KMSAN experts, for the general guidance of
> > > kmsan_copy_page_meta vs kmsan_unpoison_memory
> >
> > Oh, sorry, I've missed the previous email.
> 
> NP, thanks for jumping in :)
> 
> >
> > copy_mc_user_highpage() is expected to copy data from the user page,
> > for which no metadata is ever allocated.
> > Therefore we just initialize the destination shadow with zeros instead
> > of copying anything.
> >
> > kmsan_copy_page_meta() is used when the metadata is copied between two
> > kernel pages, therefore it handles the cases when page->kmsan_shadow
> > is NULL for the source and destination pages.
> >
> > It might be a good idea to use kmsan_copy_page_meta() in both cases,
> > but to do that I want to better understand what happens when
> > kmap_local_page(from) is called in copy_mc_user_highpage().
> > Where does the corresponding struct page come from?
> 
> Kirill can correct me, but I think khugepaged always copies user pages
> because it is trying to convert raw pages to THP for better userspace
> application performance. Therefore khugepaged should only need
> copy_mc_user_highpage(), for both file-backed and anonymous memory
> pages.
> 
> However, copy_mc_user_highpage() needs both vaddr and vma, so it is a
> little bit hard to use it in collapse_file (i.e. in the file-backed
> case):
> 1. vma is not carried over to collapse_file from khugepaged_scan_mm_slot
> 2. collapse_file is not directly iterating with vaddrs of pages to be copied
> 
> (Although both vaddr and vma are unused auguments in
> copy_mc_user_highpage, I think for cleanness, the caller e.g.
> khugepaged should feed valid values).

It is not unused for !copy_mc_to_kernel case (basically everything but x86
and power). And it is used to flush caches. The fact that you don't have
it in your implementation *may* indicate a problem.

> So my patchset uses copy_mc_page(and kmsan_copy_page_meta) for both
> file-backed and anon memory pages. I guess as long as
> kmsan_copy_page_meta doesn't do anything unexpected for user pages (at
> least from my reading), we are good?
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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

[Jiaqi Yan]

On Tue, Feb 28, 2023 at 5:40 AM <kirill@shutemov.name> wrote:
>
> On Wed, Feb 08, 2023 at 03:00:59PM -0800, Jiaqi Yan wrote:
> > On Wed, Feb 8, 2023 at 3:45 AM Alexander Potapenko <glider@google.com> wrote:
> > >
> > > On Tue, Feb 7, 2023 at 7:20 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
> > > >
> > > > Pinging KMSAN experts, for the general guidance of
> > > > kmsan_copy_page_meta vs kmsan_unpoison_memory
> > >
> > > Oh, sorry, I've missed the previous email.
> >
> > NP, thanks for jumping in :)
> >
> > >
> > > copy_mc_user_highpage() is expected to copy data from the user page,
> > > for which no metadata is ever allocated.
> > > Therefore we just initialize the destination shadow with zeros instead
> > > of copying anything.
> > >
> > > kmsan_copy_page_meta() is used when the metadata is copied between two
> > > kernel pages, therefore it handles the cases when page->kmsan_shadow
> > > is NULL for the source and destination pages.
> > >
> > > It might be a good idea to use kmsan_copy_page_meta() in both cases,
> > > but to do that I want to better understand what happens when
> > > kmap_local_page(from) is called in copy_mc_user_highpage().
> > > Where does the corresponding struct page come from?
> >
> > Kirill can correct me, but I think khugepaged always copies user pages
> > because it is trying to convert raw pages to THP for better userspace
> > application performance. Therefore khugepaged should only need
> > copy_mc_user_highpage(), for both file-backed and anonymous memory
> > pages.
> >
> > However, copy_mc_user_highpage() needs both vaddr and vma, so it is a
> > little bit hard to use it in collapse_file (i.e. in the file-backed
> > case):
> > 1. vma is not carried over to collapse_file from khugepaged_scan_mm_slot
> > 2. collapse_file is not directly iterating with vaddrs of pages to be copied
> >
> > (Although both vaddr and vma are unused auguments in
> > copy_mc_user_highpage, I think for cleanness, the caller e.g.
> > khugepaged should feed valid values).
>
> It is not unused for !copy_mc_to_kernel case (basically everything but x86
> and power). And it is used to flush caches. The fact that you don't have
> it in your implementation *may* indicate a problem.

ah, I agree this looks problematic. Maybe I should give up on unifying
the copying routine used by anon and file memory:
* for anon, switch to copy_mc_user_highpage, which "works" for all
architectures, but #MC is only recoverable on x86 and powerpc.
* for shmem, extend copy_highpage to copy_mc_highpage, like how Tony
makes copy_mc_user_highpage.

>
> > So my patchset uses copy_mc_page(and kmsan_copy_page_meta) for both
> > file-backed and anon memory pages. I guess as long as
> > kmsan_copy_page_meta doesn't do anything unexpected for user pages (at
> > least from my reading), we are good?
> >
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov