linux-kernel - Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

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Message-ID: <YQwBD55FZyoY+C5D@casper.infradead.org>
Date:   Thu, 5 Aug 2021 16:17:35 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the
 proper place

On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
> We've got the expected count for anonymous page or file page by
> expected_page_refs() at the beginning of migrate_page_move_mapping(),
> thus we should move the page count validation a little forward to
> reduce duplicated code.

Please add an explanation to the changelog for why it's safe to pull
this out from under the i_pages lock.

> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/migrate.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 239b238..5559571 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -386,11 +386,10 @@ int folio_migrate_mapping(struct address_space *mapping,
>  	int expected_count = expected_page_refs(mapping, &folio->page) + extra_count;
>  	long nr = folio_nr_pages(folio);
>  
> -	if (!mapping) {
> -		/* Anonymous page without mapping */
> -		if (folio_ref_count(folio) != expected_count)
> -			return -EAGAIN;
> +	if (folio_ref_count(folio) != expected_count)
> +		return -EAGAIN;
>  
> +	if (!mapping) {
>  		/* No turning back from here */
>  		newfolio->index = folio->index;
>  		newfolio->mapping = folio->mapping;
> @@ -404,8 +403,7 @@ int folio_migrate_mapping(struct address_space *mapping,
>  	newzone = folio_zone(newfolio);
>  
>  	xas_lock_irq(&xas);
> -	if (folio_ref_count(folio) != expected_count ||
> -	    xas_load(&xas) != folio) {
> +	if (xas_load(&xas) != folio) {
>  		xas_unlock_irq(&xas);
>  		return -EAGAIN;
>  	}
> -- 
> 1.8.3.1
> 
>