linux-kernel - Re: [PATCH] mm: avoid VM_BUG_ON when try to map an anon large folio to zero page.

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Message-ID: <99d0e61b-9408-490c-8634-f44cb2487d96@redhat.com>
Date: Wed, 23 Oct 2024 20:50:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: usamaarif642@...il.com, baohua@...nel.org, cerasuolodomenico@...il.com,
 corbet@....net, hannes@...xchg.org, kernel-team@...a.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, npache@...hat.com,
 riel@...riel.com, roman.gushchin@...ux.dev, rppt@...nel.org,
 ryan.roberts@....com, ryncsn@...il.com, shakeel.butt@...ux.dev,
 willy@...radead.org, yuzhao@...gle.com
Subject: Re: [PATCH] mm: avoid VM_BUG_ON when try to map an anon large folio
 to zero page.

On 23.10.24 19:12, Zi Yan wrote:
> An anonymous large folio can be split into non order-0 folios,
> try_to_map_unused_to_zeropage() should not VM_BUG_ON compound pages but
> just return false. This fixes the crash when splitting anonymous large
> folios to non order-0 folios.
> 
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>   mm/migrate.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e950fd62607f..7ffdbe078aa7 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -206,7 +206,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>   	pte_t newpte;
>   	void *addr;
>   
> -	VM_BUG_ON_PAGE(PageCompound(page), page);
> +	if (PageCompound(page))
> +		return false;
>   	VM_BUG_ON_PAGE(!PageAnon(page), page);
>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
>   	VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);

Would read nicer in 2024 when working on folios ;)

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb