Re: [PATCH next] mm/vmscan: __isolate_lru_page_prepare clean up

[Vlastimil Babka <vbabka@suse.cz>]

On 11/22/20 3:00 PM, Alex Shi wrote:
> Thanks a lot for all comments, I picked all up and here is the v3:
> 
>  From 167131dd106a96fd08af725df850e0da6ec899af Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@linux.alibaba.com>
> Date: Fri, 20 Nov 2020 14:49:16 +0800
> Subject: [PATCH v3 next] mm/vmscan: __isolate_lru_page_prepare clean up
> 
> The function just return 2 results, so use a 'switch' to deal with its
> result is unnecessary, and simplify it to a bool func as Vlastimil
> suggested.
> 
> Also remove 'goto' by reusing list_move(), and take Matthew Wilcox's
> suggestion to update comments in function.

I wouldn't mind if the goto stayed, but it's not repeating that much 
without it (list_move() + continue, 3 times) so...

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>   include/linux/swap.h |  2 +-
>   mm/compaction.c      |  2 +-
>   mm/vmscan.c          | 68 ++++++++++++++++++++------------------------
>   3 files changed, 33 insertions(+), 39 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 596bc2f4d9b0..5bba15ac5a2e 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -356,7 +356,7 @@ extern void lru_cache_add_inactive_or_unevictable(struct page *page,
>   extern unsigned long zone_reclaimable_pages(struct zone *zone);
>   extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>   					gfp_t gfp_mask, nodemask_t *mask);
> -extern int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
> +extern bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
>   extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>   						  unsigned long nr_pages,
>   						  gfp_t gfp_mask,
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b68931854253..8d71ffebe6cb 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -988,7 +988,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   		if (unlikely(!get_page_unless_zero(page)))
>   			goto isolate_fail;
>   
> -		if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
> +		if (!__isolate_lru_page_prepare(page, isolate_mode))
>   			goto isolate_fail_put;
>   
>   		/* Try isolate the page */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c6f94e55c3fe..4d2703c43310 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1536,19 +1536,17 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>    * page:	page to consider
>    * mode:	one of the LRU isolation modes defined above
>    *
> - * returns 0 on success, -ve errno on failure.
> + * returns true on success, false on failure.
>    */
> -int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
> +bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
>   {
> -	int ret = -EBUSY;
> -
>   	/* Only take pages on the LRU. */
>   	if (!PageLRU(page))
> -		return ret;
> +		return false;
>   
>   	/* Compaction should not handle unevictable pages but CMA can do so */
>   	if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
> -		return ret;
> +		return false;
>   
>   	/*
>   	 * To minimise LRU disruption, the caller can indicate that it only
> @@ -1561,7 +1559,7 @@ int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
>   	if (mode & ISOLATE_ASYNC_MIGRATE) {
>   		/* All the caller can do on PageWriteback is block */
>   		if (PageWriteback(page))
> -			return ret;
> +			return false;
>   
>   		if (PageDirty(page)) {
>   			struct address_space *mapping;
> @@ -1577,20 +1575,20 @@ int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
>   			 * from the page cache.
>   			 */
>   			if (!trylock_page(page))
> -				return ret;
> +				return false;
>   
>   			mapping = page_mapping(page);
>   			migrate_dirty = !mapping || mapping->a_ops->migratepage;
>   			unlock_page(page);
>   			if (!migrate_dirty)
> -				return ret;
> +				return false;
>   		}
>   	}
>   
>   	if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
> -		return ret;
> +		return false;
>   
> -	return 0;
> +	return true;
>   }
>   
>   /*
> @@ -1674,35 +1672,31 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>   		 * only when the page is being freed somewhere else.
>   		 */
>   		scan += nr_pages;
> -		switch (__isolate_lru_page_prepare(page, mode)) {
> -		case 0:
> -			/*
> -			 * Be careful not to clear PageLRU until after we're
> -			 * sure the page is not being freed elsewhere -- the
> -			 * page release code relies on it.
> -			 */
> -			if (unlikely(!get_page_unless_zero(page)))
> -				goto busy;
> -
> -			if (!TestClearPageLRU(page)) {
> -				/*
> -				 * This page may in other isolation path,
> -				 * but we still hold lru_lock.
> -				 */
> -				put_page(page);
> -				goto busy;
> -			}
> -
> -			nr_taken += nr_pages;
> -			nr_zone_taken[page_zonenum(page)] += nr_pages;
> -			list_move(&page->lru, dst);
> -			break;
> +		if (!__isolate_lru_page_prepare(page, mode)) {
> +			/* It is being freed elsewhere */
> +			list_move(&page->lru, src);
> +			continue;
> +		}
> +		/*
> +		 * Be careful not to clear PageLRU until after we're
> +		 * sure the page is not being freed elsewhere -- the
> +		 * page release code relies on it.
> +		 */
> +		if (unlikely(!get_page_unless_zero(page))) {
> +			list_move(&page->lru, src);
> +			continue;
> +		}
>   
> -		default:
> -busy:
> -			/* else it is being freed elsewhere */
> +		if (!TestClearPageLRU(page)) {
> +			/* Another thread is already isolating this page */
> +			put_page(page);
>   			list_move(&page->lru, src);
> +			continue;
>   		}
> +
> +		nr_taken += nr_pages;
> +		nr_zone_taken[page_zonenum(page)] += nr_pages;
> +		list_move(&page->lru, dst);
>   	}
>   
>   	/*
>