Re: [PATCH v5 1/4] mm/migration: make isolate_movable_page() return int type

On Wed 01-02-17 15:48:21, Minchan Kim wrote:

Hi Yisheng,

On Tue, Jan 31, 2017 at 09:06:18PM +0800, ysxie@foxmail.com wrote:

quoted

From: Yisheng Xie <redacted>

This patch changes the return type of isolate_movable_page()
from bool to int. It will return 0 when isolate movable page
successfully, return -EINVAL when the page is not a non-lru movable
page, and for other cases it will return -EBUSY.

There is no functional change within this patch but prepare
for later patch.

Signed-off-by: Yisheng Xie <redacted>
Suggested-by: Michal Hocko <mhocko@kernel.org>

Sorry for missing this one you guys were discussing.
I don't understand the patch's goal although I read later patches.

The point is that the failed isolation has to propagate error up the
call chain to the userspace which has initiated the migration.

isolate_movable_pages returns success/fail so that's why I selected
bool rather than int but it seems you guys want to propagate more
detailed error to the user so added -EBUSY and -EINVAL.

But the question is why isolate_lru_pages doesn't have -EINVAL?

It doesn't have to same as isolate_movable_pages. We should just return
EBUSY when the page is no longer movable.

Secondly, madvise man page should update?

Why?

Thirdly, if a driver fail isolation due to -ENOMEM, it should be
propagated, too?

Yes

if we want to propagte detailed error to user, driver's isolate_page
function should return right error.

Yes

I don't feel this all changes should be done now. What's the problem
if we change isolate_lru_page from int to bool? it returns just binary
value so it should be right place to use bool. If it fails, error val
is just -EBUSY.

We really want to propagate the reason why the offline operation has
failed. Why would we want to postpone that?

quoted

Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <redacted>
CC: Vlastimil Babka <redacted>
---
 include/linux/migrate.h |  2 +-
 mm/compaction.c         |  2 +-
 mm/migrate.c            | 11 +++++++----
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index ae8d475..43d5deb 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h

@@ -37,7 +37,7 @@ extern int migrate_page(struct address_space *,
 			struct page *, struct page *, enum migrate_mode);
 extern int migrate_pages(struct list_head *l, new_page_t new, free_page_t free,
 		unsigned long private, enum migrate_mode mode, int reason);
-extern bool isolate_movable_page(struct page *page, isolate_mode_t mode);
+extern int isolate_movable_page(struct page *page, isolate_mode_t mode);
 extern void putback_movable_page(struct page *page);
 
 extern int migrate_prep(void);

diff --git a/mm/compaction.c b/mm/compaction.c
index 949198d..1d89147 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c

@@ -802,7 +802,7 @@ static bool too_many_isolated(struct zone *zone)
 					locked = false;
 				}
 
-				if (isolate_movable_page(page, isolate_mode))
+				if (!isolate_movable_page(page, isolate_mode))
 					goto isolate_success;
 			}
 

diff --git a/mm/migrate.c b/mm/migrate.c
index 87f4d0f..bbbd170 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c

@@ -74,8 +74,9 @@ int migrate_prep_local(void)
 	return 0;
 }
 
-bool isolate_movable_page(struct page *page, isolate_mode_t mode)
+int isolate_movable_page(struct page *page, isolate_mode_t mode)
 {
+	int ret = -EBUSY;
 	struct address_space *mapping;
 
 	/*

@@ -95,8 +96,10 @@ bool isolate_movable_page(struct page *page, isolate_mode_t mode)
 	 * assumes anybody doesn't touch PG_lock of newly allocated page
 	 * so unconditionally grapping the lock ruins page's owner side.
 	 */
-	if (unlikely(!__PageMovable(page)))
+	if (unlikely(!__PageMovable(page))) {
+		ret = -EINVAL;
 		goto out_putpage;
+	}
 	/*
 	 * As movable pages are not isolated from LRU lists, concurrent
 	 * compaction threads can race against page migration functions

@@ -125,14 +128,14 @@ bool isolate_movable_page(struct page *page, isolate_mode_t mode)
 	__SetPageIsolated(page);
 	unlock_page(page);
 
-	return true;
+	return 0;
 
 out_no_isolated:
 	unlock_page(page);
 out_putpage:
 	put_page(page);
 out:
-	return false;
+	return ret;
 }
 
 /* It should be called on page which is PG_movable */

-- 
1.9.1

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-- 
Michal Hocko
SUSE Labs

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