Re: [PATCH 2/2] mm: ksm: do not block on page lock when searching stable tree

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 07.11.2018 22:16, Yang Shi wrote:
> ksmd need search stable tree to look for the suitable KSM page, but the
> KSM page might be locked for long time due to i.e. KSM page rmap walk.
> 
> It sounds not worth waiting for the lock, the page can be skip, then try
> to merge it in the next scan to avoid long stall if its content is
> still intact.
> 
> Introduce async mode to get_ksm_page() to not block on page lock, like
> what try_to_merge_one_page() does.
> 
> Return -EBUSY if trylock fails, since NULL means not find suitable KSM
> page, which is a valid case.
> 
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

> ---
>  mm/ksm.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 5b0894b..576803d 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -667,7 +667,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
>  }
>  
>  /*
> - * get_ksm_page: checks if the page indicated by the stable node
> + * __get_ksm_page: checks if the page indicated by the stable node
>   * is still its ksm page, despite having held no reference to it.
>   * In which case we can trust the content of the page, and it
>   * returns the gotten page; but if the page has now been zapped,
> @@ -685,7 +685,8 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
>   * a page to put something that might look like our key in page->mapping.
>   * is on its way to being freed; but it is an anomaly to bear in mind.
>   */
> -static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
> +static struct page *__get_ksm_page(struct stable_node *stable_node,
> +				   bool lock_it, bool async)
>  {
>  	struct page *page;
>  	void *expected_mapping;
> @@ -728,7 +729,14 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
>  	}
>  
>  	if (lock_it) {
> -		lock_page(page);
> +		if (async) {
> +			if (!trylock_page(page)) {
> +				put_page(page);
> +				return ERR_PTR(-EBUSY);
> +			}
> +		} else
> +			lock_page(page);
> +
>  		if (READ_ONCE(page->mapping) != expected_mapping) {
>  			unlock_page(page);
>  			put_page(page);
> @@ -751,6 +759,11 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
>  	return NULL;
>  }
>  
> +static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
> +{
> +	return __get_ksm_page(stable_node, lock_it, false);
> +}
> +
>  /*
>   * Removing rmap_item from stable or unstable tree.
>   * This function will clean the information from the stable/unstable tree.
> @@ -1675,7 +1688,11 @@ static struct page *stable_tree_search(struct page *page)
>  			 * It would be more elegant to return stable_node
>  			 * than kpage, but that involves more changes.
>  			 */
> -			tree_page = get_ksm_page(stable_node_dup, true);
> +			tree_page = __get_ksm_page(stable_node_dup, true, true);
> +
> +			if (PTR_ERR(tree_page) == -EBUSY)
> +				return ERR_PTR(-EBUSY);
> +
>  			if (unlikely(!tree_page))
>  				/*
>  				 * The tree may have been rebalanced,
> @@ -2062,6 +2079,10 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
>  
>  	/* We first start with searching the page inside the stable tree */
>  	kpage = stable_tree_search(page);
> +
> +	if (PTR_ERR(kpage) == -EBUSY)
> +		return;
> +
>  	if (kpage == page && rmap_item->head == stable_node) {
>  		put_page(kpage);
>  		return;
>