Re: [PATCH v2 1/1] mm/ksm: recover from memory failure on KSM page by migrating to healthy duplicate

[David Hildenbrand <david@redhat.com>]

On 16.10.25 12:18, Longlong Xia wrote:
> From: Longlong Xia <xialonglong@kylinos.cn>
> 
> When a hardware memory error occurs on a KSM page, the current
> behavior is to kill all processes mapping that page. This can
> be overly aggressive when KSM has multiple duplicate pages in
> a chain where other duplicates are still healthy.
> 
> This patch introduces a recovery mechanism that attempts to
> migrate mappings from the failing KSM page to a newly
> allocated KSM page or another healthy duplicate already
> present in the same chain, before falling back to the
> process-killing procedure.
> 
> The recovery process works as follows:
> 1. Identify if the failing KSM page belongs to a stable node chain.
> 2. Locate a healthy duplicate KSM page within the same chain.
> 3. For each process mapping the failing page:
>     a. Attempt to allocate a new KSM page copy from healthy duplicate
>        KSM page. If successful, migrate the mapping to this new KSM page.
>     b. If allocation fails, migrate the mapping to the existing healthy
>        duplicate KSM page.
> 4. If all migrations succeed, remove the failing KSM page from the chain.
> 5. Only if recovery fails (e.g., no healthy duplicate found or migration
>     error) does the kernel fall back to killing the affected processes.
> 
> Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>
> ---
>   mm/ksm.c | 246 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 246 insertions(+)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 160787bb121c..9099bad1ab35 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -3084,6 +3084,246 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
>   }
>   
>   #ifdef CONFIG_MEMORY_FAILURE
> +static struct ksm_stable_node *find_chain_head(struct ksm_stable_node *dup_node)
> +{
> +	struct ksm_stable_node *stable_node, *dup;
> +	struct rb_node *node;
> +	int nid;
> +
> +	if (!is_stable_node_dup(dup_node))
> +		return NULL;
> +
> +	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
> +		node = rb_first(root_stable_tree + nid);
> +		for (; node; node = rb_next(node)) {
> +			stable_node = rb_entry(node,
> +					struct ksm_stable_node,
> +					node);

Put that into a single line for readability, please.

You can also consider factoring out this inner loop in a helper function.

> +
> +			if (!is_stable_node_chain(stable_node))
> +				continue;
> +
> +			hlist_for_each_entry(dup, &stable_node->hlist,
> +					hlist_dup) {

Single line, or properly indent.

> +				if (dup == dup_node)
> +					return stable_node;
> +			}
> +		}
> +	}
> +
> +	return NULL;
> +}
> +
> +static struct folio *find_healthy_folio(struct ksm_stable_node *chain_head,
> +		struct ksm_stable_node *failing_node,
> +		struct ksm_stable_node **healthy_dupdup)
> +{
> +	struct ksm_stable_node *dup;
> +	struct hlist_node *hlist_safe;
> +	struct folio *healthy_folio;
> +
> +	if (!is_stable_node_chain(chain_head) || !is_stable_node_dup(failing_node))
> +		return NULL;
> +
> +	hlist_for_each_entry_safe(dup, hlist_safe, &chain_head->hlist, hlist_dup) {
> +		if (dup == failing_node)
> +			continue;
> +
> +		healthy_folio = ksm_get_folio(dup, KSM_GET_FOLIO_TRYLOCK);
> +		if (healthy_folio) {
> +			*healthy_dupdup = dup;
> +			return healthy_folio;
> +		}
> +	}
> +
> +	return NULL;
> +}
> +
> +static struct page *create_new_stable_node_dup(struct ksm_stable_node *chain_head,
> +		struct folio *healthy_folio,
> +		struct ksm_stable_node **new_stable_node)
> +{
> +	int nid;
> +	unsigned long kpfn;
> +	struct page *new_page = NULL;
> +
> +	if (!is_stable_node_chain(chain_head))
> +		return NULL;
> +
> +	new_page = alloc_page(GFP_HIGHUSER_MOVABLE | __GFP_ZERO);
> +	if (!new_page)
> +		return NULL;
> +
> +	copy_highpage(new_page, folio_page(healthy_folio, 0));
> +
> +	*new_stable_node = alloc_stable_node();
> +	if (!*new_stable_node) {
> +		__free_page(new_page);
> +		return NULL;
> +	}
> +
> +	INIT_HLIST_HEAD(&(*new_stable_node)->hlist);
> +	kpfn = page_to_pfn(new_page);
> +	(*new_stable_node)->kpfn = kpfn;
> +	nid = get_kpfn_nid(kpfn);
> +	DO_NUMA((*new_stable_node)->nid = nid);
> +	(*new_stable_node)->rmap_hlist_len = 0;
> +
> +	(*new_stable_node)->head = STABLE_NODE_DUP_HEAD;
> +	hlist_add_head(&(*new_stable_node)->hlist_dup, &chain_head->hlist);
> +	ksm_stable_node_dups++;
> +	folio_set_stable_node(page_folio(new_page), *new_stable_node);
> +	folio_add_lru(page_folio(new_page));

There seems to be a lot of copy-paste. For example, why no reuse 
stable_node_chain_add_dup()?

Or why not try to reuse stable_tree_insert() in the first place?

Try to reuse or factor out instead of copy-pasting, please.

> +
> +	return new_page;
> +}
> +
> +static int replace_failing_page(struct vm_area_struct *vma, struct page *page,
> +		struct page *kpage, unsigned long addr)
> +{
> +	struct folio *kfolio = page_folio(kpage);
> +	struct mm_struct *mm = vma->vm_mm;
> +	struct folio *folio = page_folio(page);
> +	pmd_t *pmd;
> +	pte_t *ptep;
> +	pte_t newpte;
> +	spinlock_t *ptl;
> +	int err = -EFAULT;
> +	struct mmu_notifier_range range;
> +
> +	pmd = mm_find_pmd(mm, addr);
> +	if (!pmd)
> +		goto out;
> +
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr,
> +			addr + PAGE_SIZE);
> +	mmu_notifier_invalidate_range_start(&range);
> +
> +	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
> +	if (!ptep)
> +		goto out_mn;
> +
> +	folio_get(kfolio);
> +	folio_add_anon_rmap_pte(kfolio, kpage, vma, addr, RMAP_NONE);
> +	newpte = mk_pte(kpage, vma->vm_page_prot);
> +
> +	flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
> +	ptep_clear_flush(vma, addr, ptep);
> +	set_pte_at(mm, addr, ptep, newpte);
> +
> +	folio_remove_rmap_pte(folio, page, vma);
> +	if (!folio_mapped(folio))
> +		folio_free_swap(folio);
> +	folio_put(folio);
> +
> +	pte_unmap_unlock(ptep, ptl);
> +	err = 0;
> +out_mn:
> +	mmu_notifier_invalidate_range_end(&range);
> +out:
> +	return err;
> +}

This is a lot of copy-paste from replace_page(). Isn't there a way to 
avoid this duplication by unifying both functions in some way?

> +
> +static void migrate_to_target_dup(struct ksm_stable_node *failing_node,
> +		struct folio *failing_folio,
> +		struct folio *target_folio,
> +		struct ksm_stable_node *target_dup)
> +{
> +	struct ksm_rmap_item *rmap_item;
> +	struct hlist_node *hlist_safe;
> +	int err;
> +
> +	hlist_for_each_entry_safe(rmap_item, hlist_safe, &failing_node->hlist, hlist) {
> +		struct mm_struct *mm = rmap_item->mm;
> +		unsigned long addr = rmap_item->address & PAGE_MASK;

Can be const.

> +		struct vm_area_struct *vma;
> +
> +		if (!mmap_read_trylock(mm))
> +			continue;
> +
> +		if (ksm_test_exit(mm)) {
> +			mmap_read_unlock(mm);
> +			continue;
> +		}
> +
> +		vma = vma_lookup(mm, addr);
> +		if (!vma) {
> +			mmap_read_unlock(mm);
> +			continue;
> +		}
> +
> +		if (!folio_trylock(target_folio)) {

Can't we leave the target folio locked the whole time? The caller 
already locked it, why not keep it locked until we're done?

> +			mmap_read_unlock(mm);
> +			continue;
> +		}
> +
> +		err = replace_failing_page(vma, &failing_folio->page,
> +				folio_page(target_folio, 0), addr);
> +		if (!err) {
> +			hlist_del(&rmap_item->hlist);
> +			rmap_item->head = target_dup;
> +			hlist_add_head(&rmap_item->hlist, &target_dup->hlist);
> +			target_dup->rmap_hlist_len++;
> +			failing_node->rmap_hlist_len--;
> +		}
> +
> +		folio_unlock(target_folio);
> +		mmap_read_unlock(mm);
> +	}
> +
> +}
> +
> +static bool ksm_recover_within_chain(struct ksm_stable_node *failing_node)
> +{
> +	struct folio *failing_folio = NULL;
> +	struct ksm_stable_node *healthy_dupdup = NULL;
> +	struct folio *healthy_folio = NULL;
> +	struct ksm_stable_node *chain_head = NULL;
> +	struct page *new_page = NULL;
> +	struct ksm_stable_node *new_stable_node = NULL;

Only initialize what needs initialization (nothing in here?) and combine 
where possible.

Like

	struct folio *failing_folio, *healthy_folio;


> +
> +	if (!is_stable_node_dup(failing_node))
> +		return false;
> +
> +	guard(mutex)(&ksm_thread_mutex);
> +	failing_folio = ksm_get_folio(failing_node, KSM_GET_FOLIO_NOLOCK);
> +	if (!failing_folio)
> +		return false;
> +
> +	chain_head = find_chain_head(failing_node);
> +	if (!chain_head)
> +		return NULL;
> +
> +	healthy_folio = find_healthy_folio(chain_head, failing_node, &healthy_dupdup);
> +	if (!healthy_folio) {
> +		folio_put(failing_folio);
> +		return false;
> +	}
> +
> +	new_page = create_new_stable_node_dup(chain_head, healthy_folio, &new_stable_node);

Why are you returning a page here and not a folio?


-- 
Cheers

David / dhildenb