linux-kernel - Re: [PATCH] mm/khugepaged: Fix race with folio splitting in hpage_collapse_scan

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Message-ID: <25cb53a2-f90e-4a70-9541-2aee51d71561@arm.com>
Date: Thu, 22 May 2025 15:37:47 +0530
From: Dev Jain <dev.jain@....com>
To: Shivank Garg <shivankg@....com>, akpm@...ux-foundation.org,
 david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, fengwei.yin@...el.com, bharata@....com,
 syzbot+2b99589e33edbe9475ca@...kaller.appspotmail.com
Subject: Re: [PATCH] mm/khugepaged: Fix race with folio splitting in
 hpage_collapse_scan_file()


On 22/05/25 3:04 pm, Shivank Garg wrote:
> folio_mapcount() checks folio_test_large() before proceeding to


It is not very clear in the description, where is this folio_mapcount() 
call present?

Are you talking about is_refcount_suitable()?


> folio_large_mapcount(), but there exists a race window where a folio
> could be split between these checks which triggered the
> VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) in
> folio_large_mapcount().
>
> Take a temporary folio reference in hpage_collapse_scan_file() to prevent
> races with concurrent folio splitting/freeing. This prevent potential
> incorrect large folio detection.
>
> Reported-by: syzbot+2b99589e33edbe9475ca@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Shivank Garg <shivankg@....com>
> ---
>   mm/khugepaged.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index cc945c6ab3bd..6e8902f9d88c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2295,6 +2295,17 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   			continue;
>   		}
>   
> +		if (!folio_try_get(folio)) {
> +			xas_reset(&xas);
> +			continue;
> +		}
> +
> +		if (unlikely(folio != xas_reload(&xas))) {
> +			folio_put(folio);
> +			xas_reset(&xas);
> +			continue;
> +		}
> +
>   		if (folio_order(folio) == HPAGE_PMD_ORDER &&
>   		    folio->index == start) {
>   			/* Maybe PMD-mapped */
> @@ -2305,23 +2316,27 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   			 * it's safe to skip LRU and refcount checks before
>   			 * returning.
>   			 */
> +			folio_put(folio);
>   			break;
>   		}
>   
>   		node = folio_nid(folio);
>   		if (hpage_collapse_scan_abort(node, cc)) {
>   			result = SCAN_SCAN_ABORT;
> +			folio_put(folio);
>   			break;
>   		}
>   		cc->node_load[node]++;
>   
>   		if (!folio_test_lru(folio)) {
>   			result = SCAN_PAGE_LRU;
> +			folio_put(folio);
>   			break;
>   		}
>   
>   		if (!is_refcount_suitable(folio)) {
>   			result = SCAN_PAGE_COUNT;
> +			folio_put(folio);
>   			break;
>   		}
>   
> @@ -2333,6 +2348,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
>   		 */
>   
>   		present += folio_nr_pages(folio);
> +		folio_put(folio);
>   
>   		if (need_resched()) {
>   			xas_pause(&xas);