We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.
If our folio has a stable node, it is a (small) KSM folio -- see
folio_stable_node(). Let's use folio_mapcount() in stable_tree_search()
instead, which results in no functional change.
The mapcount > 1 check is a bit confusing, because that's usually a check
for page sharing. Looks like the reason is that we are guaranteed to not
exceed ksm_max_page_sharing for the tree KSM folio when merging with
that. Let's update the documentation to make that clearer.
Cc: Andrew Morton <[email protected]>
Cc: Alex Shi (tencent) <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
---
mm/ksm.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 486c9974f8e20..159604ad47799 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1909,12 +1909,15 @@ static struct page *stable_tree_search(struct page *page)
if (page_node) {
VM_BUG_ON(page_node->head != &migrate_nodes);
/*
- * Test if the migrated page should be merged
- * into a stable node dup. If the mapcount is
- * 1 we can migrate it with another KSM page
- * without adding it to the chain.
+ * If the mapcount of our migrated KSM folio is
+ * at most 1, we can merge it with another
+ * KSM folio where we know that we have space
+ * for one more mapping without exceeding the
+ * ksm_max_page_sharing limit: see
+ * chain_prune(). This way, we can avoid adding
+ * this stable node to the chain.
*/
- if (page_mapcount(page) > 1)
+ if (folio_mapcount(folio) > 1)
goto chain_append;
}
--
2.44.0
On 4/17/24 1:25 AM, David Hildenbrand wrote:
> We want to limit the use of page_mapcount() to the places where it is
> absolutely necessary.
>
> If our folio has a stable node, it is a (small) KSM folio -- see
> folio_stable_node(). Let's use folio_mapcount() in stable_tree_search()
> instead, which results in no functional change.
>
> The mapcount > 1 check is a bit confusing, because that's usually a check
> for page sharing. Looks like the reason is that we are guaranteed to not
> exceed ksm_max_page_sharing for the tree KSM folio when merging with
> that. Let's update the documentation to make that clearer.
>
> Cc: Andrew Morton <[email protected]>
> Cc: Alex Shi (tencent) <[email protected]>
> Signed-off-by: David Hildenbrand <[email protected]>
> ---
> mm/ksm.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 486c9974f8e20..159604ad47799 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1909,12 +1909,15 @@ static struct page *stable_tree_search(struct page *page)
> if (page_node) {
> VM_BUG_ON(page_node->head != &migrate_nodes);
> /*
> - * Test if the migrated page should be merged
> - * into a stable node dup. If the mapcount is
> - * 1 we can migrate it with another KSM page
> - * without adding it to the chain.
> + * If the mapcount of our migrated KSM folio is
> + * at most 1, we can merge it with another
> + * KSM folio where we know that we have space
> + * for one more mapping without exceeding the
> + * ksm_max_page_sharing limit: see
> + * chain_prune(). This way, we can avoid adding
> + * this stable node to the chain.
> */
> - if (page_mapcount(page) > 1)
> + if (folio_mapcount(folio) > 1)
> goto chain_append;
> }
>
LGTM
Reviewed-by: Alex Shi <[email protected]>