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

[Minchan Kim <minchan@kernel.org>]

Hi Yisheng,

On Tue, Jan 31, 2017 at 09:06:18PM +0800, ysxie@foxmail.com wrote:
> From: Yisheng Xie <xieyisheng1@huawei.com>
> 
> 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 <xieyisheng1@huawei.com>
> 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.

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?
Secondly, madvise man page should update?
Thirdly, if a driver fail isolation due to -ENOMEM, it should be
propagated, too?

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

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.


> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> CC: Vlastimil Babka <vbabka@suse.cz>
> ---
>  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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