linux-kernel - Re: [PATCH hotfix] mm/thp: fix deferred split queue not partially

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Message-ID: <683dc253-8dd0-4990-944c-e20a3f79074c@gmail.com>
Date: Sun, 10 Nov 2024 21:22:50 +0000
From: Usama Arif <usamaarif642@...il.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>,
 Wei Yang <richard.weiyang@...il.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Matthew Wilcox <willy@...radead.org>, David Hildenbrand <david@...hat.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Barry Song <baohua@...nel.org>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Ryan Roberts
 <ryan.roberts@....com>, Nhat Pham <nphamcs@...il.com>,
 Chris Li <chrisl@...nel.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH hotfix] mm/thp: fix deferred split queue not
 partially_mapped: fix

On 10/11/2024 21:11, Hugh Dickins wrote:
> Though even more elusive than before, list_del corruption has still been
> seen on THP's deferred split queue.
> 
> The idea in commit e66f3185fa04 was right, but its implementation wrong.
> The context omitted an important comment just before the critical test:
> "split_folio() removes folio from list on success."  In ignoring that
> comment, when a THP split succeeded, the code went on to release the
> preceding safe folio, preserving instead an irrelevant (formerly head)
> folio: which gives no safety because it's not on the list. Fix the logic.
> 
> Fixes: e66f3185fa04 ("mm/thp: fix deferred split queue not partially_mapped")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Acked-by: Usama Arif <usamaarif642@...il.com>

> ---
>  mm/huge_memory.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 03fd4bc39ea1..5734d5d5060f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3790,7 +3790,9 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  		 * in the case it was underused, then consider it used and
>  		 * don't add it back to split_queue.
>  		 */
> -		if (!did_split && !folio_test_partially_mapped(folio)) {
> +		if (did_split) {
> +			; /* folio already removed from list */
> +		} else if (!folio_test_partially_mapped(folio)) {
>  			list_del_init(&folio->_deferred_list);
>  			removed++;
>  		} else {