Re: [v2 PATCH] mm: ksm: do not block on page lock when searching stable tree

[John Hubbard]

On 1/23/19 3:52 PM, Yang Shi wrote:
> ksmd need search stable tree to look for the suitable KSM page, but the
> KSM page might be locked for a while due to i.e. KSM page rmap walk.
> Basically it is not a big deal since commit 2c653d0ee2ae
> ("ksm: introduce ksm_max_page_sharing per page deduplication limit"),
> since max_page_sharing limits the number of shared KSM pages.
> 
> But it still sounds not worth waiting for the lock, the page can be skip,
> then try to merge it in the next scan to avoid potential 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.
> 
> With the default max_page_sharing setting (256), there is almost no
> observed change comparing lock vs trylock.
> 
> However, with ksm02 of LTP, the reduced ksmd full scan time can be
> observed, which has set max_page_sharing to 786432.  With lock version,
> ksmd may tak 10s - 11s to run two full scans, with trylock version ksmd
> may take 8s - 11s to run two full scans.  And, the number of
> pages_sharing and pages_to_scan keep same.  Basically, this change has
> no harm.
> 
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> Hi folks,
> 
> This patch was with "mm: vmscan: skip KSM page in direct reclaim if priority
> is low" in the initial submission.  Then Hugh and Andrea pointed out commit
> 2c653d0ee2ae ("ksm: introduce ksm_max_page_sharing per page deduplication
> limit") is good enough for limiting the number of shared KSM page to prevent
> from softlock when walking ksm page rmap.  This commit does solve the problem.
> So, the series was dropped by Andrew from -mm tree.
> 
> However, I thought the second patch (this one) still sounds useful.  So, I did
> some test and resubmit it.  The first version was reviewed by Krill Tkhai, so
> I keep his Reviewed-by tag since there is no change to the patch except the
> commit log.
> 
> So, would you please reconsider this patch?
> 
> v2: Updated the commit log to reflect some test result and latest discussion
> 
>  mm/ksm.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 6c48ad1..f66405c 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -668,7 +668,7 @@ 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,
> @@ -686,7 +686,8 @@ 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;
> @@ -729,7 +730,14 @@ 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);
> @@ -752,6 +760,11 @@ 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.
> @@ -1673,7 +1686,11 @@ 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);

Hi Yang,

The bools are stacking up: now you've got two, and the above invocation is no longer
understandable on its own. At this point, we normally shift to flags and/or an
enum.

Also, I see little value in adding a stub function here, so how about something more
like the following approximation (untested, and changes to callers are not shown):

diff --git a/mm/ksm.c b/mm/ksm.c
index 6c48ad13b4c9..8390b7905b44 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -667,6 +667,12 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
        free_stable_node(stable_node);
 }
 
+typedef enum {
+       GET_KSM_PAGE_NORMAL,
+       GET_KSM_PAGE_LOCK_PAGE,
+       GET_KSM_PAGE_TRYLOCK_PAGE
+} get_ksm_page_t;
+
 /*
  * get_ksm_page: checks if the page indicated by the stable node
  * is still its ksm page, despite having held no reference to it.
@@ -686,7 +692,8 @@ 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,
+                                get_ksm_page_t flags)
 {
        struct page *page;
        void *expected_mapping;
@@ -728,8 +735,17 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
                goto stale;
        }
 
-       if (lock_it) {
+       if (flags == GET_KSM_PAGE_TRYLOCK_PAGE) {
+               if (!trylock_page(page)) {
+                       put_page(page);
+                       return ERR_PTR(-EBUSY);
+               }
+       } else if (flags == GET_KSM_PAGE_LOCK_PAGE) {
                lock_page(page);
+       }
+
+       if (flags == GET_KSM_PAGE_LOCK_PAGE ||
+           flags == GET_KSM_PAGE_TRYLOCK_PAGE) {
                if (READ_ONCE(page->mapping) != expected_mapping) {
                        unlock_page(page);
                        put_page(page);


thanks,
-- 
John Hubbard
NVIDIA 

> +
> +			if (PTR_ERR(tree_page) == -EBUSY)
> +				return ERR_PTR(-EBUSY);
> +
>  			if (unlikely(!tree_page))
>  				/*
>  				 * The tree may have been rebalanced,
> @@ -2060,6 +2077,10 @@ 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;
>