Re: [v4 PATCH] mm: move_pages: report the number of non-attempted pages

[Wei Yang <richardw.yang@linux.intel.com>]

On Tue, Jan 28, 2020 at 05:28:47AM +0800, Yang Shi wrote:
>Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
>the semantic of move_pages() has changed to return the number of
>non-migrated pages if they were result of a non-fatal reasons (usually a
>busy page).  This was an unintentional change that hasn't been noticed
>except for LTP tests which checked for the documented behavior.
>
>There are two ways to go around this change.  We can even get back to the
>original behavior and return -EAGAIN whenever migrate_pages is not able
>to migrate pages due to non-fatal reasons.  Another option would be to
>simply continue with the changed semantic and extend move_pages
>documentation to clarify that -errno is returned on an invalid input or
>when migration simply cannot succeed (e.g. -ENOMEM, -EBUSY) or the
>number of pages that couldn't have been migrated due to ephemeral
>reasons (e.g. page is pinned or locked for other reasons).
>
>This patch implements the second option because this behavior is in
>place for some time without anybody complaining and possibly new users
>depending on it.  Also it allows to have a slightly easier error handling
>as the caller knows that it is worth to retry when err > 0.
>
>But since the new semantic would be aborted immediately if migration is
>failed due to ephemeral reasons, need include the number of non-attempted
>pages in the return value too.
>
>Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move")
>Suggested-by: Michal Hocko <mhocko@suse.com>
>Acked-by: Michal Hocko <mhocko@suse.com>
>Cc: Wei Yang <richardw.yang@linux.intel.com>
>Cc: <stable@vger.kernel.org>    [4.17+]
>Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>

>---
>v4: Fixed some typo and grammar errors caught by Willy
>v3: Rephrased the commit log per Michal and added Michal's Acked-by
>v2: Rebased on top of the latest mainline kernel per Andrew
>
> mm/migrate.c | 25 +++++++++++++++++++++++--
> 1 file changed, 23 insertions(+), 2 deletions(-)
>
>diff --git a/mm/migrate.c b/mm/migrate.c
>index 86873b6..2530860 100644
>--- a/mm/migrate.c
>+++ b/mm/migrate.c
>@@ -1627,8 +1627,19 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> 			start = i;
> 		} else if (node != current_node) {
> 			err = do_move_pages_to_node(mm, &pagelist, current_node);
>-			if (err)
>+			if (err) {
>+				/*
>+				 * Positive err means the number of failed
>+				 * pages to migrate.  Since we are going to
>+				 * abort and return the number of non-migrated
>+				 * pages, so need to incude the rest of the
>+				 * nr_pages that have not been attempted as
>+				 * well.
>+				 */
>+				if (err > 0)
>+					err += nr_pages - i - 1;
> 				goto out;
>+			}
> 			err = store_status(status, start, current_node, i - start);
> 			if (err)
> 				goto out;
>@@ -1659,8 +1670,11 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> 			goto out_flush;
> 
> 		err = do_move_pages_to_node(mm, &pagelist, current_node);
>-		if (err)
>+		if (err) {
>+			if (err > 0)
>+				err += nr_pages - i - 1;
> 			goto out;
>+		}
> 		if (i > start) {
> 			err = store_status(status, start, current_node, i - start);
> 			if (err)
>@@ -1674,6 +1688,13 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
> 
> 	/* Make sure we do not overwrite the existing error */
> 	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
>+	/*
>+	 * Don't have to report non-attempted pages here since:
>+	 *     - If the above loop is done gracefully all pages have been
>+	 *       attempted.
>+	 *     - If the above loop is aborted it means a fatal error
>+	 *       happened, should return ret.
>+	 */
> 	if (!err1)
> 		err1 = store_status(status, start, current_node, i - start);
> 	if (!err)
>-- 
>1.8.3.1

-- 
Wei Yang
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