Re: [PATCH hotfix 1/2] mm/thp: fix deferred split queue not partially_mapped

[Usama Arif]

On 24/10/2024 05:10, Hugh Dickins wrote:
> Recent changes are putting more pressure on THP deferred split queues:
> under load revealing long-standing races, causing list_del corruptions,
> "Bad page state"s and worse (I keep BUGs in both of those, so usually
> don't get to see how badly they end up without).  The relevant recent
> changes being 6.8's mTHP, 6.10's mTHP swapout, and 6.12's mTHP swapin,
> improved swap allocation, and underused THP splitting.
> 
> The new unlocked list_del_init() in deferred_split_scan() is buggy.
> I gave bad advice, it looks plausible since that's a local on-stack
> list, but the fact is that it can race with a third party freeing or
> migrating the preceding folio (properly unqueueing it with refcount 0
> while holding split_queue_lock), thereby corrupting the list linkage.
> 
> The obvious answer would be to take split_queue_lock there: but it has
> a long history of contention, so I'm reluctant to add to that. Instead,
> make sure that there is always one safe (raised refcount) folio before,
> by delaying its folio_put().  (And of course I was wrong to suggest
> updating split_queue_len without the lock: leave that until the splice.)
> 

Thanks for this, I imagine this was quite tricky to debug.

Avoiding taking the split_queue_lock by keeping reference of the
preceding folio is really nice.

And I should have spotted in the original patch that split_queue_len
shouldn't be changed without holding split_queue_lock.

Acked-by: Usama Arif <usamaarif642@xxxxxxxxx>

> And remove two over-eager partially_mapped checks, restoring those tests
> to how they were before: if uncharge_folio() or free_tail_page_prepare()
> finds _deferred_list non-empty, it's in trouble whether or not that folio
> is partially_mapped (and the flag was already cleared in the latter case).
> 
> Fixes: dafff3f4c850 ("mm: split underused THPs")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 21 +++++++++++++++++----
>  mm/memcontrol.c  |  3 +--
>  mm/page_alloc.c  |  5 ++---
>  3 files changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2fb328880b50..a1d345f1680c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3718,8 +3718,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
>  	unsigned long flags;
>  	LIST_HEAD(list);
> -	struct folio *folio, *next;
> -	int split = 0;
> +	struct folio *folio, *next, *prev = NULL;
> +	int split = 0, removed = 0;
>  
>  #ifdef CONFIG_MEMCG
>  	if (sc->memcg)
> @@ -3775,15 +3775,28 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  		 */
>  		if (!did_split && !folio_test_partially_mapped(folio)) {
>  			list_del_init(&folio->_deferred_list);
> -			ds_queue->split_queue_len--;
> +			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);
>  		}
> -		folio_put(folio);
> +		if (folio)
> +			folio_put(folio);
>  	}
>  
>  	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);
> +
>  	/*
>  	 * Stop shrinker if we didn't split any page, but the queue is empty.
>  	 * This can happen if pages were freed under us.
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7845c64a2c57..2703227cce88 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4631,8 +4631,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
>  	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
>  	VM_BUG_ON_FOLIO(folio_order(folio) > 1 &&
>  			!folio_test_hugetlb(folio) &&
> -			!list_empty(&folio->_deferred_list) &&
> -			folio_test_partially_mapped(folio), folio);
> +			!list_empty(&folio->_deferred_list), folio);
>  
>  	/*
>  	 * Nobody should be changing or seriously looking at
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8afab64814dc..4b21a368b4e2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -961,9 +961,8 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
>  		break;
>  	case 2:
>  		/* the second tail page: deferred_list overlaps ->mapping */
> -		if (unlikely(!list_empty(&folio->_deferred_list) &&
> -		    folio_test_partially_mapped(folio))) {
> -			bad_page(page, "partially mapped folio on deferred list");
> +		if (unlikely(!list_empty(&folio->_deferred_list))) {
> +			bad_page(page, "on deferred list");
>  			goto out;
>  		}
>  		break;