[PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

Ryan Roberts
2023-10-25T14:46:06Z

As preparation for supporting small-sized THP in the swap-out path,
without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
which, when present, always implies PMD-sized THP, which is the same as
the cluster size.

The only use of the flag was to determine whether a swap entry refers to
a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
Instead of relying on the flag, we now pass in nr_pages, which
originates from the folio's number of pages. This allows the logic to
work for folios of any order.

The one snag is that one of the swap_page_trans_huge_swapped() call
sites does not have the folio. But it was only being called there to
avoid bothering to call __try_to_reclaim_swap() in some cases.
__try_to_reclaim_swap() gets the folio and (via some other functions)
calls swap_page_trans_huge_swapped(). So I've removed the problematic
call site and believe the new logic should be equivalent.

Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster()
which used to be called during folio splitting, since
split_swap_cluster()'s only job was to remove the flag.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
include/linux/swap.h | 10 ----------
mm/huge_memory.c | 3 ---
mm/swapfile.c | 47 ++++++++------------------------------------
3 files changed, 8 insertions(+), 52 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 19f30a29e1f1..a073366a227c 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -259,7 +259,6 @@ struct swap_cluster_info {
};
#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
-#define CLUSTER_FLAG_HUGE 4 /* This cluster is backing a transparent huge page */

/*
* We assign a cluster to each CPU, so each CPU can allocate swap entry from
@@ -595,15 +594,6 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
}
#endif /* CONFIG_SWAP */

-#ifdef CONFIG_THP_SWAP
-extern int split_swap_cluster(swp_entry_t entry);
-#else
-static inline int split_swap_cluster(swp_entry_t entry)
-{
- return 0;
-}
-#endif
-
#ifdef CONFIG_MEMCG
static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
{
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f31f02472396..b411dd4f1612 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2598,9 +2598,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
shmem_uncharge(head->mapping->host, nr_dropped);
remap_page(folio, nr);

- if (folio_test_swapcache(folio))
- split_swap_cluster(folio->swap);
-
for (i = 0; i < nr; i++) {
struct page *subpage = head + i;
if (subpage == page)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e52f486834eb..b83ad77e04c0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -342,18 +342,6 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
info->data = 0;
}

-static inline bool cluster_is_huge(struct swap_cluster_info *info)
-{
- if (IS_ENABLED(CONFIG_THP_SWAP))
- return info->flags & CLUSTER_FLAG_HUGE;
- return false;
-}
-
-static inline void cluster_clear_huge(struct swap_cluster_info *info)
-{
- info->flags &= ~CLUSTER_FLAG_HUGE;
-}
-
static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
unsigned long offset)
{
@@ -1021,7 +1009,7 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
offset = idx * SWAPFILE_CLUSTER;
ci = lock_cluster(si, offset);
alloc_cluster(si, idx);
- cluster_set_count_flag(ci, SWAPFILE_CLUSTER, CLUSTER_FLAG_HUGE);
+ cluster_set_count(ci, SWAPFILE_CLUSTER);

memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
unlock_cluster(ci);
@@ -1354,7 +1342,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)

ci = lock_cluster_or_swap_info(si, offset);
if (size == SWAPFILE_CLUSTER) {
- VM_BUG_ON(!cluster_is_huge(ci));
map = si->swap_map + offset;
for (i = 0; i < SWAPFILE_CLUSTER; i++) {
val = map[i];
@@ -1362,7 +1349,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
if (val == SWAP_HAS_CACHE)
free_entries++;
}
- cluster_clear_huge(ci);
if (free_entries == SWAPFILE_CLUSTER) {
unlock_cluster_or_swap_info(si, ci);
spin_lock(&si->lock);
@@ -1384,23 +1370,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
unlock_cluster_or_swap_info(si, ci);
}

-#ifdef CONFIG_THP_SWAP
-int split_swap_cluster(swp_entry_t entry)
-{
- struct swap_info_struct *si;
- struct swap_cluster_info *ci;
- unsigned long offset = swp_offset(entry);
-
- si = _swap_info_get(entry);
- if (!si)
- return -EBUSY;
- ci = lock_cluster(si, offset);
- cluster_clear_huge(ci);
- unlock_cluster(ci);
- return 0;
-}
-#endif
-
static int swp_entry_cmp(const void *ent1, const void *ent2)
{
const swp_entry_t *e1 = ent1, *e2 = ent2;
@@ -1508,22 +1477,23 @@ int swp_swapcount(swp_entry_t entry)
}

static bool swap_page_trans_huge_swapped(struct swap_info_struct *si,
- swp_entry_t entry)
+ swp_entry_t entry,
+ unsigned int nr_pages)
{
struct swap_cluster_info *ci;
unsigned char *map = si->swap_map;
unsigned long roffset = swp_offset(entry);
- unsigned long offset = round_down(roffset, SWAPFILE_CLUSTER);
+ unsigned long offset = round_down(roffset, nr_pages);
int i;
bool ret = false;

ci = lock_cluster_or_swap_info(si, offset);
- if (!ci || !cluster_is_huge(ci)) {
+ if (!ci || nr_pages == 1) {
if (swap_count(map[roffset]))
ret = true;
goto unlock_out;
}
- for (i = 0; i < SWAPFILE_CLUSTER; i++) {
+ for (i = 0; i < nr_pages; i++) {
if (swap_count(map[offset + i])) {
ret = true;
break;
@@ -1545,7 +1515,7 @@ static bool folio_swapped(struct folio *folio)
if (!IS_ENABLED(CONFIG_THP_SWAP) || likely(!folio_test_large(folio)))
return swap_swapcount(si, entry) != 0;

- return swap_page_trans_huge_swapped(si, entry);
+ return swap_page_trans_huge_swapped(si, entry, folio_nr_pages(folio));
}

/**
@@ -1606,8 +1576,7 @@ int free_swap_and_cache(swp_entry_t entry)
p = _swap_info_get(entry);
if (p) {
count = __swap_entry_free(p, entry);
- if (count == SWAP_HAS_CACHE &&
- !swap_page_trans_huge_swapped(p, entry))
+ if (count == SWAP_HAS_CACHE)
__try_to_reclaim_swap(p, swp_offset(entry),
TTRS_UNMAPPED | TTRS_FULL);
}
--
2.25.1