Subject: Re: [RFC 07/11] mm: remove SWAP_AGAIN in ttu
To: Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
References: <1488436765-32350-1-git-send-email-minchan@kernel.org>
 <1488436765-32350-8-git-send-email-minchan@kernel.org>
Cc: kernel-team@lge.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.com>
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Date: Fri, 3 Mar 2017 18:24:06 +0530
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On 03/02/2017 12:09 PM, Minchan Kim wrote:
> In 2002, [1] introduced SWAP_AGAIN.
> At that time, ttuo used spin_trylock(&mm->page_table_lock) so it's

Small nit: Please expand "ttuo" here. TTU in the first place is also
not very clear but we have that in many places.

> really easy to contend and fail to hold a lock so SWAP_AGAIN to keep
> LRU status makes sense.

Okay.

> 
> However, now we changed it to mutex-based lock and be able to block
> without skip pte so there is a few of small window to return
> SWAP_AGAIN so remove SWAP_AGAIN and just return SWAP_FAIL.

Makes sense.

> 
> [1] c48c43e, minimal rmap
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/rmap.c   | 11 +++--------
>  mm/vmscan.c |  2 --
>  2 files changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 47898a1..da18f21 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1492,13 +1492,10 @@ static int page_mapcount_is_zero(struct page *page)
>   * Return values are:
>   *
>   * SWAP_SUCCESS	- we succeeded in removing all mappings
> - * SWAP_AGAIN	- we missed a mapping, try again later
>   * SWAP_FAIL	- the page is unswappable
>   */
>  int try_to_unmap(struct page *page, enum ttu_flags flags)
>  {
> -	int ret;
> -
>  	struct rmap_walk_control rwc = {
>  		.rmap_one = try_to_unmap_one,
>  		.arg = (void *)flags,
> @@ -1518,13 +1515,11 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
>  		rwc.invalid_vma = invalid_migration_vma;
>  
>  	if (flags & TTU_RMAP_LOCKED)
> -		ret = rmap_walk_locked(page, &rwc);
> +		rmap_walk_locked(page, &rwc);
>  	else
> -		ret = rmap_walk(page, &rwc);
> +		rmap_walk(page, &rwc);
>  
> -	if (!page_mapcount(page))
> -		ret = SWAP_SUCCESS;
> -	return ret;
> +	return !page_mapcount(page) ? SWAP_SUCCESS: SWAP_FAIL;

Its very simple now. So after the rmap_walk() if page is not mapped any
more return SWAP_SUCCESS otherwise SWAP_FAIL.