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Subject: Re: [Patch v2 4/4] mm/migrate.c: handle same node and add failure in
 the same way
To:     Wei Yang <richardw.yang@linux.intel.com>, akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org, mhocko@suse.com,
        yang.shi@linux.alibaba.com, rientjes@google.com
References: <20200122011647.13636-1-richardw.yang@linux.intel.com>
 <20200122011647.13636-5-richardw.yang@linux.intel.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <17796dbb-b9b6-9481-5048-addfa5eec51e@redhat.com>
Date:   Wed, 29 Jan 2020 11:13:23 +0100
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In-Reply-To: <20200122011647.13636-5-richardw.yang@linux.intel.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On 22.01.20 02:16, Wei Yang wrote:
> When page is not queued for migration, there are two possible cases:
>=20
>   * page already on the target node
>   * failed to add to migration queue
>=20
> Current code handle them differently, this leads to a behavior
> inconsistency.
>=20
> Usually for each page's status, we just do store for once. While for th=
e
> page already on the target node, we might store the node information fo=
r
> twice:
>=20
>   * once when we found the page is on the target node
>   * second when moving the pages to target node successfully after abov=
e
>     action
>=20
> The reason is even we don't add the page to pagelist, but store_status(=
)
> does store in a range which still contains the page.
>=20
> This patch handles these two cases in the same way to reduce this
> inconsistency and also make the code a little easier to read.
>=20

I'd rephrase to

"mm/migrate.c: unify "not queued for migration" handling in do_pages_move=
()

It can currently happen that we store the status of a page twice:
* Once we detect that it is already on the target node
* Once we moved a bunch of pages, and a page that's already on the
  target node is contained in the current interval.

Let's simplify the code and always call do_move_pages_to_node() in
case we did not queue a page for migration. Note that pages that are
already on the target node are not added to the pagelist and are,
therefore, ignored by do_move_pages_to_node() - there is no functional
change.

The status of such a page is now only stored once.
"

> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/migrate.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>=20
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 80d2bba57265..591f2e5caed6 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1654,18 +1654,18 @@ static int do_pages_move(struct mm_struct *mm, =
nodemask_t task_nodes,
>  		err =3D add_page_for_migration(mm, addr, current_node,
>  				&pagelist, flags & MPOL_MF_MOVE_ALL);
> =20
> -		if (!err) {
> -			/* The page is already on the target node */
> -			err =3D store_status(status, i, current_node, 1);
> -			if (err)
> -				goto out_flush;
> -			continue;
> -		} else if (err > 0) {
> +		if (err > 0) {
>  			/* The page is successfully queued for migration */
>  			continue;
>  		}
> =20
> -		err =3D store_status(status, i, err, 1);
> +		/*
> +		 * Two possible cases for err here:
> +		 * =3D=3D 0: page is already on the target node, then store
> +		 *       current_node to status
> +		 * <  0: failed to add page to list, then store err to status
> +		 */

I'd shorten that to

/*
 * If the page is already on the target node (!err), store the node,
 * otherwise, store the err.
*/

> +		err =3D store_status(status, i, err ? : current_node, 1);
>  		if (err)
>  			goto out_flush;
> =20
>=20

Thanks!

Reviewed-by: David Hildenbrand <david@redhat.com>

--=20
Thanks,

David / dhildenb