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From: Chris Li <chrisl@kernel.org>
Date: Fri, 24 May 2024 10:17:18 -0700
Subject: [PATCH 1/2] mm: swap: swap cluster switch to double link list
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Message-Id: <20240524-swap-allocator-v1-1-47861b423b26@kernel.org>
References: <20240524-swap-allocator-v1-0-47861b423b26@kernel.org>
In-Reply-To: <20240524-swap-allocator-v1-0-47861b423b26@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kairui Song <kasong@tencent.com>, Ryan Roberts <ryan.roberts@arm.com>, 
 "Huang, Ying" <ying.huang@intel.com>, linux-kernel@vger.kernel.org, 
 linux-mm@kvack.org, Chris Li <chrisl@kernel.org>, 
 Barry Song <baohua@kernel.org>

Previously, the swap cluster used a cluster index as a pointer
to construct a custom single link list type "swap_cluster_list".
The next cluster pointer is shared with the cluster->count.
The assumption is that only the free cluster needs to be put
on the list.

That assumption is not true for mTHP allocators any more. Need
to track the non full cluster on the list as well.  Move the
current cluster single link list into standard double link list.

Remove the cluster getter/setter for accessing the cluster
struct member.  Move the cluster locking in the caller function
rather than the getter/setter function. That way the locking can
protect more than one member, e.g. cluster->flag.

Change cluster code to use "struct swap_cluster_info *" to
reference the cluster rather than by using index. That is more
consistent with the list manipulation. It avoids the repeat
adding index to the cluser_info. The code is easier to understand.

Remove the cluster next pointer is NULL flag, the double link
list can handle the empty list pretty well.

The "swap_cluster_info" struct is two pointer bigger, because
512 swap entries share one swap struct, it has very little impact
on the average memory usage per swap entry.  Other than the list
conversion, there is no real function change in this patch.
---
 include/linux/swap.h |  14 ++--
 mm/swapfile.c        | 231 ++++++++++++++-------------------------------------
 2 files changed, 68 insertions(+), 177 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 11c53692f65f..0d3906eff3c9 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -254,11 +254,12 @@ struct swap_cluster_info {
 				 * elements correspond to the swap
 				 * cluster
 				 */
-	unsigned int data:24;
+	unsigned int count:16;
 	unsigned int flags:8;
+	struct list_head next;
 };
 #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
-#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
+
 
 /*
  * The first page in the swap file is the swap header, which is always marked
@@ -283,11 +284,6 @@ struct percpu_cluster {
 	unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
 };
 
-struct swap_cluster_list {
-	struct swap_cluster_info head;
-	struct swap_cluster_info tail;
-};
-
 /*
  * The in-memory structure used to track swap areas.
  */
@@ -300,7 +296,7 @@ struct swap_info_struct {
 	unsigned int	max;		/* extent of the swap_map */
 	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
 	struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
-	struct swap_cluster_list free_clusters; /* free clusters list */
+	struct list_head free_clusters; /* free clusters list */
 	unsigned int lowest_bit;	/* index of first free in swap_map */
 	unsigned int highest_bit;	/* index of last free in swap_map */
 	unsigned int pages;		/* total of usable pages of swap */
@@ -333,7 +329,7 @@ struct swap_info_struct {
 					 * list.
 					 */
 	struct work_struct discard_work; /* discard worker */
-	struct swap_cluster_list discard_clusters; /* discard clusters list */
+	struct list_head discard_clusters; /* discard clusters list */
 	struct plist_node avail_lists[]; /*
 					   * entries in swap_avail_heads, one
 					   * entry per node.
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4f0e8b2ac8aa..205a60c5f9cb 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -290,64 +290,11 @@ static void discard_swap_cluster(struct swap_info_struct *si,
 #endif
 #define LATENCY_LIMIT		256
 
-static inline void cluster_set_flag(struct swap_cluster_info *info,
-	unsigned int flag)
-{
-	info->flags = flag;
-}
-
-static inline unsigned int cluster_count(struct swap_cluster_info *info)
-{
-	return info->data;
-}
-
-static inline void cluster_set_count(struct swap_cluster_info *info,
-				     unsigned int c)
-{
-	info->data = c;
-}
-
-static inline void cluster_set_count_flag(struct swap_cluster_info *info,
-					 unsigned int c, unsigned int f)
-{
-	info->flags = f;
-	info->data = c;
-}
-
-static inline unsigned int cluster_next(struct swap_cluster_info *info)
-{
-	return info->data;
-}
-
-static inline void cluster_set_next(struct swap_cluster_info *info,
-				    unsigned int n)
-{
-	info->data = n;
-}
-
-static inline void cluster_set_next_flag(struct swap_cluster_info *info,
-					 unsigned int n, unsigned int f)
-{
-	info->flags = f;
-	info->data = n;
-}
-
 static inline bool cluster_is_free(struct swap_cluster_info *info)
 {
 	return info->flags & CLUSTER_FLAG_FREE;
 }
 
-static inline bool cluster_is_null(struct swap_cluster_info *info)
-{
-	return info->flags & CLUSTER_FLAG_NEXT_NULL;
-}
-
-static inline void cluster_set_null(struct swap_cluster_info *info)
-{
-	info->flags = CLUSTER_FLAG_NEXT_NULL;
-	info->data = 0;
-}
-
 static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
 						     unsigned long offset)
 {
@@ -394,65 +341,11 @@ static inline void unlock_cluster_or_swap_info(struct swap_info_struct *si,
 		spin_unlock(&si->lock);
 }
 
-static inline bool cluster_list_empty(struct swap_cluster_list *list)
-{
-	return cluster_is_null(&list->head);
-}
-
-static inline unsigned int cluster_list_first(struct swap_cluster_list *list)
-{
-	return cluster_next(&list->head);
-}
-
-static void cluster_list_init(struct swap_cluster_list *list)
-{
-	cluster_set_null(&list->head);
-	cluster_set_null(&list->tail);
-}
-
-static void cluster_list_add_tail(struct swap_cluster_list *list,
-				  struct swap_cluster_info *ci,
-				  unsigned int idx)
-{
-	if (cluster_list_empty(list)) {
-		cluster_set_next_flag(&list->head, idx, 0);
-		cluster_set_next_flag(&list->tail, idx, 0);
-	} else {
-		struct swap_cluster_info *ci_tail;
-		unsigned int tail = cluster_next(&list->tail);
-
-		/*
-		 * Nested cluster lock, but both cluster locks are
-		 * only acquired when we held swap_info_struct->lock
-		 */
-		ci_tail = ci + tail;
-		spin_lock_nested(&ci_tail->lock, SINGLE_DEPTH_NESTING);
-		cluster_set_next(ci_tail, idx);
-		spin_unlock(&ci_tail->lock);
-		cluster_set_next_flag(&list->tail, idx, 0);
-	}
-}
-
-static unsigned int cluster_list_del_first(struct swap_cluster_list *list,
-					   struct swap_cluster_info *ci)
-{
-	unsigned int idx;
-
-	idx = cluster_next(&list->head);
-	if (cluster_next(&list->tail) == idx) {
-		cluster_set_null(&list->head);
-		cluster_set_null(&list->tail);
-	} else
-		cluster_set_next_flag(&list->head,
-				      cluster_next(&ci[idx]), 0);
-
-	return idx;
-}
-
 /* Add a cluster to discard list and schedule it to do discard */
 static void swap_cluster_schedule_discard(struct swap_info_struct *si,
-		unsigned int idx)
+		struct swap_cluster_info *ci)
 {
+	unsigned int idx = ci - si->cluster_info;
 	/*
 	 * If scan_swap_map_slots() can't find a free cluster, it will check
 	 * si->swap_map directly. To make sure the discarding cluster isn't
@@ -462,17 +355,16 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
 	memset(si->swap_map + idx * SWAPFILE_CLUSTER,
 			SWAP_MAP_BAD, SWAPFILE_CLUSTER);
 
-	cluster_list_add_tail(&si->discard_clusters, si->cluster_info, idx);
-
+	spin_lock_nested(&ci->lock, SINGLE_DEPTH_NESTING);
+	list_add_tail(&ci->next, &si->discard_clusters);
+	spin_unlock(&ci->lock);
 	schedule_work(&si->discard_work);
 }
 
-static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
+static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
 {
-	struct swap_cluster_info *ci = si->cluster_info;
-
-	cluster_set_flag(ci + idx, CLUSTER_FLAG_FREE);
-	cluster_list_add_tail(&si->free_clusters, ci, idx);
+	ci->flags = CLUSTER_FLAG_FREE;
+	list_add_tail(&ci->next, &si->free_clusters);
 }
 
 /*
@@ -481,21 +373,21 @@ static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
 */
 static void swap_do_scheduled_discard(struct swap_info_struct *si)
 {
-	struct swap_cluster_info *info, *ci;
+	struct swap_cluster_info *ci;
 	unsigned int idx;
 
-	info = si->cluster_info;
-
-	while (!cluster_list_empty(&si->discard_clusters)) {
-		idx = cluster_list_del_first(&si->discard_clusters, info);
+	while (!list_empty(&si->discard_clusters)) {
+		ci = list_first_entry(&si->discard_clusters, struct swap_cluster_info, next);
+		idx = ci - si->cluster_info;
 		spin_unlock(&si->lock);
 
 		discard_swap_cluster(si, idx * SWAPFILE_CLUSTER,
 				SWAPFILE_CLUSTER);
 
 		spin_lock(&si->lock);
-		ci = lock_cluster(si, idx * SWAPFILE_CLUSTER);
-		__free_cluster(si, idx);
+
+		spin_lock(&ci->lock);
+		__free_cluster(si, ci);
 		memset(si->swap_map + idx * SWAPFILE_CLUSTER,
 				0, SWAPFILE_CLUSTER);
 		unlock_cluster(ci);
@@ -521,20 +413,20 @@ static void swap_users_ref_free(struct percpu_ref *ref)
 	complete(&si->comp);
 }
 
-static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
+static struct swap_cluster_info *alloc_cluster(struct swap_info_struct *si, unsigned long idx)
 {
-	struct swap_cluster_info *ci = si->cluster_info;
+	struct swap_cluster_info *ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, next);
 
-	VM_BUG_ON(cluster_list_first(&si->free_clusters) != idx);
-	cluster_list_del_first(&si->free_clusters, ci);
-	cluster_set_count_flag(ci + idx, 0, 0);
+	VM_BUG_ON(ci - si->cluster_info != idx);
+	list_del(&ci->next);
+	ci->count = 0;
+	ci->flags = 0;
+	return ci;
 }
 
-static void free_cluster(struct swap_info_struct *si, unsigned long idx)
+static void free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
 {
-	struct swap_cluster_info *ci = si->cluster_info + idx;
-
-	VM_BUG_ON(cluster_count(ci) != 0);
+	VM_BUG_ON(ci->count != 0);
 	/*
 	 * If the swap is discardable, prepare discard the cluster
 	 * instead of free it immediately. The cluster will be freed
@@ -542,11 +434,11 @@ static void free_cluster(struct swap_info_struct *si, unsigned long idx)
 	 */
 	if ((si->flags & (SWP_WRITEOK | SWP_PAGE_DISCARD)) ==
 	    (SWP_WRITEOK | SWP_PAGE_DISCARD)) {
-		swap_cluster_schedule_discard(si, idx);
+		swap_cluster_schedule_discard(si, ci);
 		return;
 	}
 
-	__free_cluster(si, idx);
+	__free_cluster(si, ci);
 }
 
 /*
@@ -559,15 +451,15 @@ static void add_cluster_info_page(struct swap_info_struct *p,
 	unsigned long count)
 {
 	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
+	struct swap_cluster_info *ci = cluster_info + idx;
 
 	if (!cluster_info)
 		return;
-	if (cluster_is_free(&cluster_info[idx]))
+	if (cluster_is_free(ci))
 		alloc_cluster(p, idx);
 
-	VM_BUG_ON(cluster_count(&cluster_info[idx]) + count > SWAPFILE_CLUSTER);
-	cluster_set_count(&cluster_info[idx],
-		cluster_count(&cluster_info[idx]) + count);
+	VM_BUG_ON(ci->count + count > SWAPFILE_CLUSTER);
+	ci->count += count;
 }
 
 /*
@@ -581,24 +473,20 @@ static void inc_cluster_info_page(struct swap_info_struct *p,
 }
 
 /*
- * The cluster corresponding to page_nr decreases one usage. If the usage
- * counter becomes 0, which means no page in the cluster is in using, we can
- * optionally discard the cluster and add it to free cluster list.
+ * The cluster ci decreases one usage. If the usage counter becomes 0,
+ * which means no page in the cluster is in using, we can optionally discard
+ * the cluster and add it to free cluster list.
  */
-static void dec_cluster_info_page(struct swap_info_struct *p,
-	struct swap_cluster_info *cluster_info, unsigned long page_nr)
+static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluster_info *ci)
 {
-	unsigned long idx = page_nr / SWAPFILE_CLUSTER;
-
-	if (!cluster_info)
+	if (!p->cluster_info)
 		return;
 
-	VM_BUG_ON(cluster_count(&cluster_info[idx]) == 0);
-	cluster_set_count(&cluster_info[idx],
-		cluster_count(&cluster_info[idx]) - 1);
+	VM_BUG_ON(ci->count == 0);
+	ci->count--;
 
-	if (cluster_count(&cluster_info[idx]) == 0)
-		free_cluster(p, idx);
+	if (!ci->count)
+		free_cluster(p, ci);
 }
 
 /*
@@ -611,10 +499,10 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
 {
 	struct percpu_cluster *percpu_cluster;
 	bool conflict;
-
+	struct swap_cluster_info *first = list_first_entry(&si->free_clusters, struct swap_cluster_info, next);
 	offset /= SWAPFILE_CLUSTER;
-	conflict = !cluster_list_empty(&si->free_clusters) &&
-		offset != cluster_list_first(&si->free_clusters) &&
+	conflict = !list_empty(&si->free_clusters) &&
+		offset !=  first - si->cluster_info &&
 		cluster_is_free(&si->cluster_info[offset]);
 
 	if (!conflict)
@@ -655,10 +543,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
 	cluster = this_cpu_ptr(si->percpu_cluster);
 	tmp = cluster->next[order];
 	if (tmp == SWAP_NEXT_INVALID) {
-		if (!cluster_list_empty(&si->free_clusters)) {
-			tmp = cluster_next(&si->free_clusters.head) *
-					SWAPFILE_CLUSTER;
-		} else if (!cluster_list_empty(&si->discard_clusters)) {
+		if (!list_empty(&si->free_clusters)) {
+			ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, next);
+			list_del(&ci->next);
+			spin_lock(&ci->lock);
+			ci->flags = 0;
+			spin_unlock(&ci->lock);
+			tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER;
+		} else if (!list_empty(&si->discard_clusters)) {
 			/*
 			 * we don't have free cluster but have some clusters in
 			 * discarding, do discard now and reclaim them, then
@@ -670,7 +562,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
 			goto new_cluster;
 		} else
 			return false;
-	}
+	} else
+		ci = si->cluster_info + tmp;
 
 	/*
 	 * Other CPUs can use our cluster if they can't find a free cluster,
@@ -1062,8 +955,9 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
 
 	ci = lock_cluster(si, offset);
 	memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER);
-	cluster_set_count_flag(ci, 0, 0);
-	free_cluster(si, idx);
+	ci->count = 0;
+	ci->flags = 0;
+	free_cluster(si, ci);
 	unlock_cluster(ci);
 	swap_range_free(si, offset, SWAPFILE_CLUSTER);
 }
@@ -1336,7 +1230,7 @@ static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
 	count = p->swap_map[offset];
 	VM_BUG_ON(count != SWAP_HAS_CACHE);
 	p->swap_map[offset] = 0;
-	dec_cluster_info_page(p, p->cluster_info, offset);
+	dec_cluster_info_page(p, ci);
 	unlock_cluster(ci);
 
 	mem_cgroup_uncharge_swap(entry, 1);
@@ -2985,8 +2879,8 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
 
 	nr_good_pages = maxpages - 1;	/* omit header page */
 
-	cluster_list_init(&p->free_clusters);
-	cluster_list_init(&p->discard_clusters);
+	INIT_LIST_HEAD(&p->free_clusters);
+	INIT_LIST_HEAD(&p->discard_clusters);
 
 	for (i = 0; i < swap_header->info.nr_badpages; i++) {
 		unsigned int page_nr = swap_header->info.badpages[i];
@@ -3037,14 +2931,15 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
 	for (k = 0; k < SWAP_CLUSTER_COLS; k++) {
 		j = (k + col) % SWAP_CLUSTER_COLS;
 		for (i = 0; i < DIV_ROUND_UP(nr_clusters, SWAP_CLUSTER_COLS); i++) {
+			struct swap_cluster_info *ci;
 			idx = i * SWAP_CLUSTER_COLS + j;
+			ci = cluster_info + idx;
 			if (idx >= nr_clusters)
 				continue;
-			if (cluster_count(&cluster_info[idx]))
+			if (ci->count)
 				continue;
-			cluster_set_flag(&cluster_info[idx], CLUSTER_FLAG_FREE);
-			cluster_list_add_tail(&p->free_clusters, cluster_info,
-					      idx);
+			ci->flags = CLUSTER_FLAG_FREE;
+			list_add_tail(&ci->next, &p->free_clusters);
 		}
 	}
 	return nr_extents;

-- 
2.45.1.288.g0e0cd299f1-goog