linux-kernel - Re: [PATCH v2 7/7] mm/migrate: remove result argument on page allocation function for migration

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Message-ID: <20131211084719.GA2043@lge.com>
Date:	Wed, 11 Dec 2013 17:47:19 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Rafael Aquini <aquini@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Vlastimil Babka <vbabka@...e.cz>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v2 7/7] mm/migrate: remove result argument on page
 allocation function for migration

On Mon, Dec 09, 2013 at 04:40:06PM +0000, Christoph Lameter wrote:
> On Mon, 9 Dec 2013, Joonsoo Kim wrote:
> 
> > First, we don't use error number in fail case. Call-path related to
> > new_page_node() is shown in the following.
> >
> > do_move_page_to_node_array() -> migrate_pages() -> unmap_and_move()
> > -> new_page_node()
> >
> > If unmap_and_move() failed, migrate_pages() also returns err, and then
> > do_move_page_to_node_array() skips to set page's status to user buffer.
> > So we don't need to set error number to each pages on failure case.
> 
> I dont get this. new_page_node() sets the error condition in the
> page_to_node array before this patch. There is no post processing in
> do_move_page_to_node_array(). The function simply returns and relies on
> new_page_node() to have set the page status. do_move_pages() then returns
> the page status back to userspace. How does the change preserve these
> diagnostics?

Hello, Christoph.

In do_move_pages(), if error occurs, 'goto out_pm' is executed and the
page status doesn't back to userspace. So we don't need to store err number.

Perhaps my description should be changed something like below.

  *do_move_pages()* -> do_move_page_to_node_array() -> migrate_pages()
  -> unmap_and_move() -> new_page_node()

  If unmap_and_move() failed, migrate_pages() also returns err, and then
  *do_move_pages()* skips to set page's status to user buffer.
  So we don't need to set error number to each pages on failure case.

Is it sufficient explanation?

> 
> > Next, we don't need to set node id of the new page in unmap_and_move(),
> > since it cannot be different with pm->node. In new_page_node(), we always
> > try to allocate the page in exact node by referencing pm->node. So it is
> > sufficient to set node id of the new page in new_page_node(), instead of
> > unmap_and_move().
> 
> Thats a good thought.

Thanks!
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