| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <x4d36plhxcbyp76q4gmesktnnh7yi7bfifx3amk3fwx2moqkk6@77umpnw6rkg3>
Date: Mon, 6 Oct 2025 16:16:25 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com,
roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...hat.com,
lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com,
baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
dev.jain@....com, baohua@...nel.org, lance.yang@...ux.dev,
akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, Muchun Song <songmuchun@...edance.com>,
Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v4 3/4] mm: thp: use folio_batch to handle THP splitting
in deferred_split_scan()
On Sat, Oct 04, 2025 at 12:53:17AM +0800, Qi Zheng wrote:
> From: Muchun Song <songmuchun@...edance.com>
>
> The maintenance of the folio->_deferred_list is intricate because it's
> reused in a local list.
>
> Here are some peculiarities:
>
> 1) When a folio is removed from its split queue and added to a local
> on-stack list in deferred_split_scan(), the ->split_queue_len isn't
> updated, leading to an inconsistency between it and the actual
> number of folios in the split queue.
>
> 2) When the folio is split via split_folio() later, it's removed from
> the local list while holding the split queue lock. At this time,
> this lock protects the local list, not the split queue.
I think the above text needs some massaging. Rather than saying lock
protects the local list, I think, it would be better to say that the
lock is not needed as it is not protecting anything.
>
> 3) To handle the race condition with a third-party freeing or migrating
> the preceding folio, we must ensure there's always one safe (with
> raised refcount) folio before by delaying its folio_put(). More
> details can be found in commit e66f3185fa04 ("mm/thp: fix deferred
> split queue not partially_mapped"). It's rather tricky.
>
> We can use the folio_batch infrastructure to handle this clearly. In this
> case, ->split_queue_len will be consistent with the real number of folios
> in the split queue. If list_empty(&folio->_deferred_list) returns false,
> it's clear the folio must be in its split queue (not in a local list
> anymore).
>
> In the future, we will reparent LRU folios during memcg offline to
> eliminate dying memory cgroups, which requires reparenting the split queue
> to its parent first. So this patch prepares for using
> folio_split_queue_lock_irqsave() as the memcg may change then.
>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Acked-by: David Hildenbrand <david@...hat.com>
One nit below.
Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> ---
> mm/huge_memory.c | 85 ++++++++++++++++++++++--------------------------
> 1 file changed, 39 insertions(+), 46 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 134666503440d..59ddebc9f3232 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3782,21 +3782,22 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
> struct lruvec *lruvec;
> int expected_refs;
>
> - if (folio_order(folio) > 1 &&
> - !list_empty(&folio->_deferred_list)) {
> - ds_queue->split_queue_len--;
> + if (folio_order(folio) > 1) {
> + if (!list_empty(&folio->_deferred_list)) {
> + ds_queue->split_queue_len--;
> + /*
> + * Reinitialize page_deferred_list after removing the
> + * page from the split_queue, otherwise a subsequent
> + * split will see list corruption when checking the
> + * page_deferred_list.
> + */
> + list_del_init(&folio->_deferred_list);
> + }
> if (folio_test_partially_mapped(folio)) {
> folio_clear_partially_mapped(folio);
> mod_mthp_stat(folio_order(folio),
> MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> }
> - /*
> - * Reinitialize page_deferred_list after removing the
> - * page from the split_queue, otherwise a subsequent
> - * split will see list corruption when checking the
> - * page_deferred_list.
> - */
> - list_del_init(&folio->_deferred_list);
> }
> split_queue_unlock(ds_queue);
> if (mapping) {
> @@ -4185,35 +4186,40 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> {
> struct deferred_split *ds_queue;
> unsigned long flags;
> - LIST_HEAD(list);
> - struct folio *folio, *next, *prev = NULL;
> - int split = 0, removed = 0;
> + struct folio *folio, *next;
> + int split = 0, i;
> + struct folio_batch fbatch;
>
> + folio_batch_init(&fbatch);
> +
> +retry:
> ds_queue = split_queue_lock_irqsave(sc->nid, sc->memcg, &flags);
> /* Take pin on all head pages to avoid freeing them under us */
> list_for_each_entry_safe(folio, next, &ds_queue->split_queue,
> _deferred_list) {
> if (folio_try_get(folio)) {
> - list_move(&folio->_deferred_list, &list);
> - } else {
> + folio_batch_add(&fbatch, folio);
> + } else if (folio_test_partially_mapped(folio)) {
> /* We lost race with folio_put() */
> - 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_del_init(&folio->_deferred_list);
> - ds_queue->split_queue_len--;
> + folio_clear_partially_mapped(folio);
> + mod_mthp_stat(folio_order(folio),
> + MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> }
> + list_del_init(&folio->_deferred_list);
> + ds_queue->split_queue_len--;
> if (!--sc->nr_to_scan)
> break;
> + if (!folio_batch_space(&fbatch))
> + break;
> }
> split_queue_unlock_irqrestore(ds_queue, flags);
>
> - list_for_each_entry_safe(folio, next, &list, _deferred_list) {
> + for (i = 0; i < folio_batch_count(&fbatch); i++) {
> bool did_split = false;
> bool underused = false;
> + struct deferred_split *fqueue;
>
> + folio = fbatch.folios[i];
> if (!folio_test_partially_mapped(folio)) {
> /*
> * See try_to_map_unused_to_zeropage(): we cannot
> @@ -4236,38 +4242,25 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> }
> folio_unlock(folio);
> next:
> + if (did_split || !folio_test_partially_mapped(folio))
> + continue;
> /*
> - * split_folio() removes folio from list on success.
> * Only add back to the queue if folio is partially mapped.
> * If thp_underused returns false, or if split_folio fails
> * in the case it was underused, then consider it used and
> * don't add it back to split_queue.
> */
> - if (did_split) {
> - ; /* folio already removed from list */
> - } else if (!folio_test_partially_mapped(folio)) {
> - list_del_init(&folio->_deferred_list);
> - removed++;
> - } else {
> - /*
> - * That unlocked list_del_init() above would be unsafe,
> - * unless its folio is separated from any earlier folios
> - * left on the list (which may be concurrently unqueued)
> - * by one safe folio with refcount still raised.
> - */
> - swap(folio, prev);
> + fqueue = folio_split_queue_lock_irqsave(folio, &flags);
> + if (list_empty(&folio->_deferred_list)) {
> + list_add_tail(&folio->_deferred_list, &fqueue->split_queue);
> + fqueue->split_queue_len++;
> }
> - if (folio)
> - folio_put(folio);
> + split_queue_unlock_irqrestore(fqueue, flags);
Is it possible to move this lock/list_add/unlock code chunk out of loop
and before the folios_put(). I think it would be possible if you tag the
corresponding index or have a separate bool array. It is also reasonable
to claim that the contention of this lock is not a concern for now.
> }
> + folios_put(&fbatch);
>
> - spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> - list_splice_tail(&list, &ds_queue->split_queue);
> - ds_queue->split_queue_len -= removed;
> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> -
> - if (prev)
> - folio_put(prev);
> + if (sc->nr_to_scan)
> + goto retry;
>
> /*
> * Stop shrinker if we didn't split any page, but the queue is empty.
> --
> 2.20.1
>
Powered by blists - more mailing lists