Re: [RFC][PATCH 1/2] mm/migrate: remove extra page_count() check

[Yang Shi]

On 5/1/20 2:05 PM, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> This is not a bug fix.  It was found by inspection, but I believe
> that it is confusing as it stands.
>
> First, page_ref_freeze() is implemented internally with:
>
> 	atomic_cmpxchg(&page->_refcount, expected, 0) == expected
>
> The "cmp" part of cmpxchg is making sure that _refcount==expected
> which means that there's an implicit check here, equivalent to:
>
> 	page_count(page) == expected_count
>
> This appears to have originated in "e286781: mm: speculative page
> references", which is pretty ancient.  This check is also somewhat
> dangerous to have here because it might lead someone to think that
> page_ref_freeze() *doesn't* do its own page_count() checking.
>
> Remove the unnecessary check.

Make sense to me. Acked-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>
>   b/mm/migrate.c |    3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff -puN mm/migrate.c~remove_extra_page_count_check mm/migrate.c
> --- a/mm/migrate.c~remove_extra_page_count_check	2020-05-01 14:00:42.331525924 -0700
> +++ b/mm/migrate.c	2020-05-01 14:00:42.336525924 -0700
> @@ -425,11 +425,12 @@ int migrate_page_move_mapping(struct add
>   	newzone = page_zone(newpage);
>   
>   	xas_lock_irq(&xas);
> -	if (page_count(page) != expected_count || xas_load(&xas) != page) {
> +	if (xas_load(&xas) != page) {
>   		xas_unlock_irq(&xas);
>   		return -EAGAIN;
>   	}
>   
> +	/* Freezing will fail if page_count()!=expected_count */
>   	if (!page_ref_freeze(page, expected_count)) {
>   		xas_unlock_irq(&xas);
>   		return -EAGAIN;
> _