Re: [RFC PATCH 16/57] mm/collapse: freeze the sources behind migration entries

[Kiryl Shutsemau <kirill@shutemov.name>]

On Mon, Aug 24, 2026 at 09:12:24PM +0800, Lance Yang wrote:
> 
> On Sun, Aug 16, 2026 at 11:45:28PM +0100, Kiryl Shutsemau wrote:
> >+		if (!folio_ref_freeze(folio,
> >+				      folio_expected_ref_count(folio) + 1)) {
> >+			result = SCAN_PAGE_COUNT;
> >+			goto unfreeze;
> >+		}
> >+		nr_frozen = nr_saved;
> 
> Just one thing I was wondering about ... can deferred_split_isolate()
> remove a source folio from deferred_split_lru while its refcount is
> frozen by collapse_freeze_candidate()?
> 
> Assume an earlier span belongs to an anonymous large folio on the
> deferred split queue, then a later span fails folio_trylock().
> collapse_freeze_candidate() continues after freezing each source folio
> and calls collapse_unfreeze_candidate() on a later failure:
> 
> static noinline enum scan_result collapse_freeze_candidate(struct mm_struct *mm,
> 		struct collapse_candidate *cand, pte_t *pte)
> {
> ...
> 	for (i = 0, addr = cand->addr; i < nr_pages;) {
> ...
> 		if (!folio_trylock(folio)) {
> 			folio_put(folio);
> 			result = SCAN_PAGE_LOCK;
> 			goto unfreeze;
> 		}
> ...
> 		nr_saved = i + nr;
> 
> 		if (!folio_ref_freeze(folio,
> 				      folio_expected_ref_count(folio) + 1)) {
> 			result = SCAN_PAGE_COUNT;
> 			goto unfreeze;
> 		}
> 		nr_frozen = nr_saved;
> 
> 		i += nr;
> 		addr += nr * PAGE_SIZE;
> 	}
> 
> ...
> unfreeze:
> 	collapse_unfreeze_candidate(mm, cand, pte, nr_saved, nr_frozen);
> 	return result;
> }
> 
> folio_ref_freeze() takes the source folio's refcount to zero:
> 
> static inline int folio_ref_freeze(struct folio *folio, int count)
> {
> 	return page_ref_freeze(&folio->page, count);
> }
> 
> static inline int page_ref_freeze(struct page *page, int count)
> {
> 	int ret = likely(atomic_cmpxchg(&page->_refcount, count, 0) == count);
> 
> ...
> 	return ret;
> }
> 
> While collapse_freeze_candidate() still holds the source folio lock,
> deferred_split_scan() can call deferred_split_isolate():
> 
> static unsigned long deferred_split_scan(struct shrinker *shrink,
> 		struct shrink_control *sc)
> {
> 	LIST_HEAD(dispose);
> 	struct folio *folio, *next;
> 	int split = 0;
> 	unsigned long isolated;
> 
> 	isolated = list_lru_shrink_walk_irq(&deferred_split_lru, sc,
> 					    deferred_split_isolate, &dispose);
> }
> 
> static enum lru_status deferred_split_isolate(struct list_head *item,
> 					      struct list_lru_one *lru,
> 					      void *cb_arg)
> {
> 	struct folio *folio = container_of(item, struct folio, _deferred_list);
> 	struct list_head *freeable = cb_arg;
> 
> 	if (folio_try_get(folio)) {
> 		list_lru_isolate_move(lru, item, freeable);
> 		return LRU_REMOVED;
> 	}
> 
> 	/*
> 	 * We lost race with folio_put(). Read folio state before the
> 	 * isolate: folio_unqueue_deferred_split() checks list_empty()
> 	 * locklessly, so once removed the folio can be freed any time.
> 	 */
> 	if (folio_test_partially_mapped(folio)) {
> 		folio_clear_partially_mapped(folio);
> 		mod_mthp_stat(folio_order(folio),
> 			      MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> 	}
> 	list_lru_isolate(lru, item);
> 	return LRU_REMOVED;
> }
> 
> And folio_try_get() fails because the source folio has a frozen refcount.
> deferred_split_isolate() treats the failure as a race with folio_put(),
> clears PG_partially_mapped and its MTHP_STAT_NR_ANON_PARTIALLY_MAPPED
> accounting when set, then removes the folio from deferred_split_lru ...

Hm. So, the premise in deferred_split_isolate() is false:
!folio_try_get() doesn't mean lost race with folio_put().

I think deferred_split_isolate() should do something like:

      if (!folio_try_get(folio))
              return LRU_SKIP;

      list_lru_isolate_move(lru, item, freeable);
      return LRU_REMOVED;

Johannes, do I miss something?

> 
> >+
> >+		i += nr;
> >+		addr += nr * PAGE_SIZE;
> >+	}
> >+
> >+	cand->state = CAND_FROZEN;
> >+	return SCAN_SUCCEED;
> >+
> >+unfreeze:
> >+	collapse_unfreeze_candidate(mm, cand, pte, nr_saved, nr_frozen);
> >+	return result;
> 
> And collapse_unfreeze_candidate() restores the source PTEs and refcount,
> then unlocks and puts the source folio :)
> 
> The source folio is not added back to the deferred split queue, so an
> underused or partially mapped folio can remain off the queue.
> 
> Should collapse unqueue the source folio after folio_ref_freeze()
> succeeds, remember whether it was on the deferred split queue and whether
> PG_partially_mapped was set, then requeue it if
> collapse_unfreeze_candidate() restores the source folio?
> 
> [...]
> 
> Cheers, Lance

-- 
  Kiryl Shutsemau / Kirill A. Shutemov