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mm/ksm: optimize the chain()/chain_prune() interfaces

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Now the implementation of stable_node_dup() causes chain()/chain_prune()
interfaces and usages are overcomplicated.

Why?  stable_node_dup() only find and return a candidate stable_node for
sharing, so the users have to recheck using stable_node_dup_any() if any
non-candidate stable_node exist.  And try to ksm_get_folio() from it
again.

Actually, stable_node_dup() can just return a best stable_node as it can,
then the users can check if it's a candidate for sharing or not.

The code is simplified too and fewer corner cases: such as stable_node and
stable_node_dup can't be NULL if returned tree_folio is not NULL.

Link: https://lkml.kernel.org/r/20240621-b4-ksm-scan-optimize-v2-3-1c328aa9e30b@linux.dev
Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Chengming Zhou 2024-06-21 15:54:31 +08:00 committed by Andrew Morton
parent d58a361b03
commit a0b856b617
1 changed files with 27 additions and 125 deletions
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134
mm/ksm.c
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@ -1659,7 +1659,6 @@ static struct folio *stable_node_dup(struct ksm_stable_node **_stable_node_dup,
struct ksm_stable_node *dup, *found = NULL, *stable_node = *_stable_node;
struct hlist_node *hlist_safe;
struct folio *folio, *tree_folio = NULL;
int nr = 0;
int found_rmap_hlist_len;
if (!prune_stale_stable_nodes ||
@ -1686,33 +1685,26 @@ static struct folio *stable_node_dup(struct ksm_stable_node **_stable_node_dup,
folio = ksm_get_folio(dup, KSM_GET_FOLIO_NOLOCK);
if (!folio)
continue;
nr += 1;
if (is_page_sharing_candidate(dup)) {
if (!found ||
dup->rmap_hlist_len > found_rmap_hlist_len) {
/* Pick the best candidate if possible. */
if (!found || (is_page_sharing_candidate(dup) &&
(!is_page_sharing_candidate(found) ||
dup->rmap_hlist_len > found_rmap_hlist_len))) {
if (found)
folio_put(tree_folio);
found = dup;
found_rmap_hlist_len = found->rmap_hlist_len;
tree_folio = folio;
/* skip put_page for found dup */
if (!prune_stale_stable_nodes)
/* skip put_page for found candidate */
if (!prune_stale_stable_nodes &&
is_page_sharing_candidate(found))
break;
continue;
}
}
folio_put(folio);
}
if (found) {
/*
* nr is counting all dups in the chain only if
* prune_stale_stable_nodes is true, otherwise we may
* break the loop at nr == 1 even if there are
* multiple entries.
*/
if (prune_stale_stable_nodes && nr == 1) {
if (hlist_is_singular_node(&found->hlist_dup, &stable_node->hlist)) {
/*
* If there's not just one entry it would
* corrupt memory, better BUG_ON. In KSM
@ -1764,25 +1756,15 @@ static struct folio *stable_node_dup(struct ksm_stable_node **_stable_node_dup,
hlist_add_head(&found->hlist_dup,
&stable_node->hlist);
}
} else {
/* Its hlist must be empty if no one found. */
free_stable_node_chain(stable_node, root);
}
*_stable_node_dup = found;
return tree_folio;
}
static struct ksm_stable_node *stable_node_dup_any(struct ksm_stable_node *stable_node,
struct rb_root *root)
{
if (!is_stable_node_chain(stable_node))
return stable_node;
if (hlist_empty(&stable_node->hlist)) {
free_stable_node_chain(stable_node, root);
return NULL;
}
return hlist_entry(stable_node->hlist.first,
typeof(*stable_node), hlist_dup);
}
/*
* Like for ksm_get_folio, this function can free the *_stable_node and
* *_stable_node_dup if the returned tree_page is NULL.
@ -1803,18 +1785,11 @@ static struct folio *__stable_node_chain(struct ksm_stable_node **_stable_node_d
bool prune_stale_stable_nodes)
{
struct ksm_stable_node *stable_node = *_stable_node;
if (!is_stable_node_chain(stable_node)) {
if (is_page_sharing_candidate(stable_node)) {
*_stable_node_dup = stable_node;
return ksm_get_folio(stable_node, KSM_GET_FOLIO_NOLOCK);
}
/*
* _stable_node_dup set to NULL means the stable_node
* reached the ksm_max_page_sharing limit.
*/
*_stable_node_dup = NULL;
return NULL;
}
return stable_node_dup(_stable_node_dup, _stable_node, root,
prune_stale_stable_nodes);
}
@ -1827,16 +1802,10 @@ static __always_inline struct folio *chain_prune(struct ksm_stable_node **s_n_d,
}
static __always_inline struct folio *chain(struct ksm_stable_node **s_n_d,
struct ksm_stable_node *s_n,
struct ksm_stable_node **s_n,
struct rb_root *root)
{
struct ksm_stable_node *old_stable_node = s_n;
struct folio *tree_folio;
tree_folio = __stable_node_chain(s_n_d, &s_n, root, false);
/* not pruning dups so s_n cannot have changed */
VM_BUG_ON(s_n != old_stable_node);
return tree_folio;
return __stable_node_chain(s_n_d, s_n, root, false);
}
/*
@ -1854,7 +1823,7 @@ static struct page *stable_tree_search(struct page *page)
struct rb_root *root;
struct rb_node **new;
struct rb_node *parent;
struct ksm_stable_node *stable_node, *stable_node_dup, *stable_node_any;
struct ksm_stable_node *stable_node, *stable_node_dup;
struct ksm_stable_node *page_node;
struct folio *folio;
@ -1878,45 +1847,7 @@ again:
cond_resched();
stable_node = rb_entry(*new, struct ksm_stable_node, node);
stable_node_any = NULL;
tree_folio = chain_prune(&stable_node_dup, &stable_node, root);
/*
* NOTE: stable_node may have been freed by
* chain_prune() if the returned stable_node_dup is
* not NULL. stable_node_dup may have been inserted in
* the rbtree instead as a regular stable_node (in
* order to collapse the stable_node chain if a single
* stable_node dup was found in it). In such case the
* stable_node is overwritten by the callee to point
* to the stable_node_dup that was collapsed in the
* stable rbtree and stable_node will be equal to
* stable_node_dup like if the chain never existed.
*/
if (!stable_node_dup) {
/*
* Either all stable_node dups were full in
* this stable_node chain, or this chain was
* empty and should be rb_erased.
*/
stable_node_any = stable_node_dup_any(stable_node,
root);
if (!stable_node_any) {
/* rb_erase just run */
goto again;
}
/*
* Take any of the stable_node dups page of
* this stable_node chain to let the tree walk
* continue. All KSM pages belonging to the
* stable_node dups in a stable_node chain
* have the same content and they're
* write protected at all times. Any will work
* fine to continue the walk.
*/
tree_folio = ksm_get_folio(stable_node_any,
KSM_GET_FOLIO_NOLOCK);
}
VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
if (!tree_folio) {
/*
* If we walked over a stale stable_node,
@ -1954,7 +1885,7 @@ again:
goto chain_append;
}
if (!stable_node_dup) {
if (!is_page_sharing_candidate(stable_node_dup)) {
/*
* If the stable_node is a chain and
* we got a payload match in memcmp
@ -2063,9 +1994,6 @@ replace:
return &folio->page;
chain_append:
/* stable_node_dup could be null if it reached the limit */
if (!stable_node_dup)
stable_node_dup = stable_node_any;
/*
* If stable_node was a chain and chain_prune collapsed it,
* stable_node has been updated to be the new regular
@ -2110,7 +2038,7 @@ static struct ksm_stable_node *stable_tree_insert(struct folio *kfolio)
struct rb_root *root;
struct rb_node **new;
struct rb_node *parent;
struct ksm_stable_node *stable_node, *stable_node_dup, *stable_node_any;
struct ksm_stable_node *stable_node, *stable_node_dup;
bool need_chain = false;
kpfn = folio_pfn(kfolio);
@ -2126,33 +2054,7 @@ again:
cond_resched();
stable_node = rb_entry(*new, struct ksm_stable_node, node);
stable_node_any = NULL;
tree_folio = chain(&stable_node_dup, stable_node, root);
if (!stable_node_dup) {
/*
* Either all stable_node dups were full in
* this stable_node chain, or this chain was
* empty and should be rb_erased.
*/
stable_node_any = stable_node_dup_any(stable_node,
root);
if (!stable_node_any) {
/* rb_erase just run */
goto again;
}
/*
* Take any of the stable_node dups page of
* this stable_node chain to let the tree walk
* continue. All KSM pages belonging to the
* stable_node dups in a stable_node chain
* have the same content and they're
* write protected at all times. Any will work
* fine to continue the walk.
*/
tree_folio = ksm_get_folio(stable_node_any,
KSM_GET_FOLIO_NOLOCK);
}
VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
tree_folio = chain(&stable_node_dup, &stable_node, root);
if (!tree_folio) {
/*
* If we walked over a stale stable_node,