[PATCH v6 0/2] Memory poison recovery in khugepaged collapsing

[PATCH v6 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
=========

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/linux/highmem.h            |  19 +++
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 233 ++++++++++++++++++++---------
 3 files changed, 183 insertions(+), 72 deletions(-)

-- 
2.38.1.431.g37b22c650d-goog

[PATCH v6 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/linux/highmem.h            |  19 ++++
 include/trace/events/huge_memory.h |   3 +-
 mm/khugepaged.c                    | 159 +++++++++++++++++++++--------
 3 files changed, 138 insertions(+), 43 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 44242268f53bd..bcd317998e4bb 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -361,6 +361,25 @@ 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/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 ea0d186bc9d4c..c1f225327bc05 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -55,6 +55,7 @@ enum scan_result {
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_COPY_MC,
 };
 
 #define CREATE_TRACE_POINTS
@@ -670,56 +671,125 @@ 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. Cleanup the normal pages if copying succeeds;
+ * otherwise restore the original page table and release isolated normal pages.
+ * 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 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 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))) {
-			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_highpage_mc(page, src_page)) {
+				copy_succeeded = false;
+				break;
+			}
+		}
+	}
+
+	if (copy_succeeded) {
+		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);
 			}
-		} 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);
+			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);
+			}
+		}
+		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 copy_succeeded;
 }
 
 static void khugepaged_alloc_sleep(void)
@@ -981,6 +1051,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	int result = SCAN_FAIL;
 	struct vm_area_struct *vma;
 	struct mmu_notifier_range range;
+	bool copied = false;
 
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
@@ -1084,9 +1155,13 @@ 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);
+	copied = __collapse_huge_page_copy(pte, hpage, 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
-- 
2.38.1.431.g37b22c650d-goog

[PATCH v6 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 thread to inspect their buffer contents and make sure no
   data corruption.

Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
---
 mm/khugepaged.c | 74 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 45 insertions(+), 29 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c1f225327bc05..d54527b77e365 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1776,7 +1776,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 			 struct collapse_control *cc)
 {
 	struct address_space *mapping = file->f_mapping;
-	struct page *hpage;
+	struct page *hpage, *page, *tmp;
 	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
@@ -1821,7 +1821,7 @@ 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);
+		page = xas_next(&xas);
 
 		VM_BUG_ON(index != xas.xa_index);
 		if (is_shmem) {
@@ -2003,10 +2003,7 @@ 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
@@ -2017,21 +2014,10 @@ 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:
@@ -2044,20 +2030,34 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	try_to_unmap_flush();
 
 	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(hpage + (index % HPAGE_PMD_NR));
 				index++;
 			}
-			copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
-				      page);
+			if (copy_highpage_mc(hpage + (page->index % HPAGE_PMD_NR), page)) {
+				result = SCAN_COPY_MC;
+				break;
+			}
+			index++;
+		}
+		while (result == SCAN_SUCCEED && index < end) {
+			clear_highpage(hpage + (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);
@@ -2065,12 +2065,23 @@ 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);
 
 		SetPageUptodate(hpage);
 		page_ref_add(hpage, HPAGE_PMD_NR - 1);
@@ -2086,8 +2097,6 @@ 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) {
@@ -2121,6 +2130,13 @@ 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;
-- 
2.38.1.431.g37b22c650d-goog

Re: [PATCH v6 0/2] Memory poison recovery in khugepaged collapsing

[Andrew Morton]

On Sun,  6 Nov 2022 18:53:57 -0800 Jiaqi Yan <jiaqiyan@google.com> wrote:

> 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.

Thanks, I'll toss v6 into mm-unstable for some testing, pending further review.

When resending a patchset, please try to also cc the people who have
commented on previous versions.

Re: [PATCH v6 0/2] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

Thanks for ccing Oscar, Andrew.

After getting this patch into our internal production environment, I
recently found a regression bug introduced by my commit a0157a2c735b
("mm/khugepaged: recover from poisoned file-backed memory").
Given it is only in mm-unstable, I wonder should I put out a v7 with
the fix, or should I make it a new and separate commit?
Sorry for the bug.


On Mon, Nov 7, 2022 at 12:53 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Sun,  6 Nov 2022 18:53:57 -0800 Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> > 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.
>
> Thanks, I'll toss v6 into mm-unstable for some testing, pending further review.
>
> When resending a patchset, please try to also cc the people who have
> commented on previous versions.
>

Re: [PATCH v6 0/2] Memory poison recovery in khugepaged collapsing

[Andrew Morton]

On Wed, 16 Nov 2022 09:58:23 -0800 Jiaqi Yan <jiaqiyan@google.com> wrote:

> Thanks for ccing Oscar, Andrew.
> 
> After getting this patch into our internal production environment, I
> recently found a regression bug introduced by my commit a0157a2c735b
> ("mm/khugepaged: recover from poisoned file-backed memory").
> Given it is only in mm-unstable, I wonder should I put out a v7 with
> the fix, or should I make it a new and separate commit?

Either approach is OK.  I usually convert replacement patches into
deltas so I and others can see what changed.  But people like to see
the whole patch for review purposes, so I guess that if you email out a
new version of the patch, we get to see both.

Re: [PATCH v6 0/2] Memory poison recovery in khugepaged collapsing

[Jiaqi Yan]

On Wed, Nov 16, 2022 at 1:52 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 16 Nov 2022 09:58:23 -0800 Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> > Thanks for ccing Oscar, Andrew.
> >
> > After getting this patch into our internal production environment, I
> > recently found a regression bug introduced by my commit a0157a2c735b
> > ("mm/khugepaged: recover from poisoned file-backed memory").
> > Given it is only in mm-unstable, I wonder should I put out a v7 with
> > the fix, or should I make it a new and separate commit?
>
> Either approach is OK.  I usually convert replacement patches into
> deltas so I and others can see what changed.  But people like to see
> the whole patch for review purposes, so I guess that if you email out a
> new version of the patch, we get to see both.
>
>

Thanks Andrew. I just sent out v7 with the fix.