linux-kernel - Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page

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Date:   Fri, 10 Jul 2020 17:31:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     robbieko <robbieko@...ology.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page

On 7/9/20 4:48 AM, robbieko wrote:
> From: Robbie Ko <robbieko@...ology.com>
> 
> When a migrate page occurs, we first create a migration entry
> to replace the original pte, and then go to fallback_migrate_page
> to execute a writeout if the migratepage is not supported.
> 
> In the writeout, we will clear the dirty bit of the page and use
> page_mkclean to clear the dirty bit along with the corresponding pte,
> but page_mkclean does not support migration entry.
> 
> The page ditry bit is cleared, but the dirty bit of the pte still exists,
> so if mmap continues to write, it will result in data loss.

Curious, did you observe this data loss? What filesystem? If yes, it seems
serious enough to
CC stable and determine a Fixes: tag?

> We fix the by first remove the migration entry and then clearing
> the dirty bits of the page, which also clears the pte's dirty bits.
> 
> Signed-off-by: Robbie Ko <robbieko@...ology.com>
> ---
>  mm/migrate.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f37729673558..5c407434b9ba 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -875,10 +875,6 @@ static int writeout(struct address_space *mapping, struct page *page)
>  		/* No write method for the address space */
>  		return -EINVAL;
>  
> -	if (!clear_page_dirty_for_io(page))
> -		/* Someone else already triggered a write */
> -		return -EAGAIN;
> -
>  	/*
>  	 * A dirty page may imply that the underlying filesystem has
>  	 * the page on some queue. So the page must be clean for
> @@ -889,6 +885,10 @@ static int writeout(struct address_space *mapping, struct page *page)
>  	 */
>  	remove_migration_ptes(page, page, false);
>  
> +	if (!clear_page_dirty_for_io(page))
> +		/* Someone else already triggered a write */
> +		return -EAGAIN;
> +
>  	rc = mapping->a_ops->writepage(page, &wbc);
>  
>  	if (rc != AOP_WRITEPAGE_ACTIVATE)
>