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

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