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SubjectRe: [v3 PATCH] mm: ksm: do not block on page lock when searching stable tree
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On 1/29/19 12:29 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 trylock mode to get_ksm_page() to not block on page lock, like
> what try_to_merge_one_page() does. And, define three possible
> operations (nolock, lock and trylock) as enum type to avoid stacking up
> bools and make the code more readable.
>
> 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@google.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Suggested-by: John Hubbard <jhubbard@nvidia.com>
> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> 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?
>
> v3: Use enum to define get_ksm_page operations (nolock, lock and trylock) per
> John Hubbard
> v2: Updated the commit log to reflect some test result and latest discussion
>
> mm/ksm.c | 46 ++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 36 insertions(+), 10 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 6c48ad1..5647bc1 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);
> }
>
> +enum get_ksm_page_flags {
> + GET_KSM_PAGE_NOLOCK,
> + GET_KSM_PAGE_LOCK,
> + GET_KSM_PAGE_TRYLOCK
> +};
> +
> /*
> * 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,
> + enum get_ksm_page_flags flags)
> {
> struct page *page;
> void *expected_mapping;
> @@ -728,8 +735,15 @@ 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) {
> + if (!trylock_page(page)) {
> + put_page(page);
> + return ERR_PTR(-EBUSY);
> + }
> + } else if (flags == GET_KSM_PAGE_LOCK)
> lock_page(page);
> +
> + if (flags != GET_KSM_PAGE_NOLOCK) {
> if (READ_ONCE(page->mapping) != expected_mapping) {
> unlock_page(page);
> put_page(page);
> @@ -763,7 +777,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> struct page *page;
>
> stable_node = rmap_item->head;
> - page = get_ksm_page(stable_node, true);
> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
> if (!page)
> goto out;
>
> @@ -863,7 +877,7 @@ static int remove_stable_node(struct stable_node *stable_node)
> struct page *page;
> int err;
>
> - page = get_ksm_page(stable_node, true);
> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
> if (!page) {
> /*
> * get_ksm_page did remove_node_from_stable_tree itself.
> @@ -1385,7 +1399,7 @@ static struct page *stable_node_dup(struct stable_node **_stable_node_dup,
> * stable_node parameter itself will be freed from
> * under us if it returns NULL.
> */
> - _tree_page = get_ksm_page(dup, false);
> + _tree_page = get_ksm_page(dup, GET_KSM_PAGE_NOLOCK);
> if (!_tree_page)
> continue;
> nr += 1;
> @@ -1508,7 +1522,7 @@ static struct page *__stable_node_chain(struct stable_node **_stable_node_dup,
> if (!is_stable_node_chain(stable_node)) {
> if (is_page_sharing_candidate(stable_node)) {
> *_stable_node_dup = stable_node;
> - return get_ksm_page(stable_node, false);
> + return get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
> }
> /*
> * _stable_node_dup set to NULL means the stable_node
> @@ -1613,7 +1627,8 @@ static struct page *stable_tree_search(struct page *page)
> * wrprotected at all times. Any will work
> * fine to continue the walk.
> */
> - tree_page = get_ksm_page(stable_node_any, false);
> + tree_page = get_ksm_page(stable_node_any,
> + GET_KSM_PAGE_NOLOCK);
> }
> VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
> if (!tree_page) {
> @@ -1673,7 +1688,12 @@ 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,
> + GET_KSM_PAGE_TRYLOCK);
> +
> + if (PTR_ERR(tree_page) == -EBUSY)
> + return ERR_PTR(-EBUSY);

or just:

if (PTR_ERR(tree_page) == -EBUSY)
return tree_page;

right?

> +
> if (unlikely(!tree_page))
> /*
> * The tree may have been rebalanced,
> @@ -1842,7 +1862,8 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
> * wrprotected at all times. Any will work
> * fine to continue the walk.
> */
> - tree_page = get_ksm_page(stable_node_any, false);
> + tree_page = get_ksm_page(stable_node_any,
> + GET_KSM_PAGE_NOLOCK);
> }
> VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
> if (!tree_page) {
> @@ -2060,6 +2081,10 @@ 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;
> @@ -2242,7 +2267,8 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
>
> list_for_each_entry_safe(stable_node, next,
> &migrate_nodes, list) {
> - page = get_ksm_page(stable_node, false);
> + page = get_ksm_page(stable_node,
> + GET_KSM_PAGE_NOLOCK);
> if (page)
> put_page(page);
> cond_resched();
>

Hi Yang,

The patch looks correct as far doing what it claims to do. I'll leave it
to others to decide if a trylock-based approach is really what you want,
for KSM scans. It seems reasonable from my very limited knowledge of
KSM: there shouldn't be any cases where you really *need* to wait for
a page lock, because the whole system is really sort of an optimization
anyway.


thanks,
--
John Hubbard
NVIDIA

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