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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Aug 24, 2026 at 03:13:10PM +0100, Kiryl Shutsemau wrote:
> 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 suppose you mean, not exclusively. But it can also mean that. And
then the question is, who cleans up the partially_mapped state.

> 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?

Ah. The idea being: leave the item on the LRU when there is a race
with the refcount going zero; and then it's up to that other side to
deal with/clean up the partially_mapped state as appropriate. Thus:

folio_put()
  __folio_put()
    folio_unqueue_deferred_split()
      if __list_lru_del():
        // clear partially_mapped state & stats

will always succeed, even if it races with the shrinker. And you are
also guaranteed on the collapse side that the state won't vanish from
underneath you.

I think that should work. Usama?