[PATCH v4 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

Ryan Roberts
2024-03-11T15:01:15Z

Now that we no longer have a convenient flag in the cluster to determine
if a folio is large, free_swap_and_cache() will take a reference and
lock a large folio much more often, which could lead to contention and
(e.g.) failure to split large folios, etc.

Let's solve that problem by batch freeing swap and cache with a new
function, free_swap_and_cache_nr(), to free a contiguous range of swap
entries together. This allows us to first drop a reference to each swap
slot before we try to release the cache folio. This means we only try to
release the folio once, only taking the reference and lock once - much
better than the previous 512 times for the 2M THP case.

Contiguous swap entries are gathered in zap_pte_range() and
madvise_free_pte_range() in a similar way to how present ptes are
already gathered in zap_pte_range().

While we are at it, let's simplify by converting the return type of both
functions to void. The return value was used only by zap_pte_range() to
print a bad pte, and was ignored by everyone else, so the extra
reporting wasn't exactly guaranteed. We will still get the warning with
most of the information from get_swap_device(). With the batch version,
we wouldn't know which pte was bad anyway so could print the wrong one.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
include/linux/pgtable.h | 28 +++++++++++++++
include/linux/swap.h | 12 +++++--
mm/internal.h | 48 +++++++++++++++++++++++++
mm/madvise.c | 12 ++++---
mm/memory.c | 13 +++----
mm/swapfile.c | 78 ++++++++++++++++++++++++++++++-----------
6 files changed, 157 insertions(+), 34 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 85fc7554cd52..8cf1f2fe2c25 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -708,6 +708,34 @@ static inline void pte_clear_not_present_full(struct mm_struct *mm,
}
#endif

+#ifndef clear_not_present_full_ptes
+/**
+ * clear_not_present_full_ptes - Clear consecutive not present PTEs.
+ * @mm: Address space the ptes represent.
+ * @addr: Address of the first pte.
+ * @ptep: Page table pointer for the first entry.
+ * @nr: Number of entries to clear.
+ * @full: Whether we are clearing a full mm.
+ *
+ * May be overridden by the architecture; otherwise, implemented as a simple
+ * loop over pte_clear_not_present_full().
+ *
+ * Context: The caller holds the page table lock. The PTEs are all not present.
+ * The PTEs are all in the same PMD.
+ */
+static inline void clear_not_present_full_ptes(struct mm_struct *mm,
+ unsigned long addr, pte_t *ptep, unsigned int nr, int full)
+{
+ for (;;) {
+ pte_clear_not_present_full(mm, addr, ptep, full);
+ if (--nr == 0)
+ break;
+ ptep++;
+ addr += PAGE_SIZE;
+ }
+}
+#endif
+
#ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
unsigned long address,
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4a8b6c60793a..f2b7f204b968 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -481,7 +481,7 @@ extern int swap_duplicate(swp_entry_t);
extern int swapcache_prepare(swp_entry_t);
extern void swap_free(swp_entry_t);
extern void swapcache_free_entries(swp_entry_t *entries, int n);
-extern int free_swap_and_cache(swp_entry_t);
+extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
int swap_type_of(dev_t device, sector_t offset);
int find_first_swap(dev_t *device);
extern unsigned int count_swap_pages(int, int);
@@ -530,8 +530,9 @@ static inline void put_swap_device(struct swap_info_struct *si)
#define free_pages_and_swap_cache(pages, nr) \
release_pages((pages), (nr));

-/* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
-#define free_swap_and_cache(e) is_pfn_swap_entry(e)
+static inline void free_swap_and_cache_nr(swp_entry_t entry, int nr)
+{
+}

static inline void free_swap_cache(struct folio *folio)
{
@@ -599,6 +600,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
}
#endif /* CONFIG_SWAP */

+static inline void free_swap_and_cache(swp_entry_t entry)
+{
+ free_swap_and_cache_nr(entry, 1);
+}
+
#ifdef CONFIG_MEMCG
static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
{
diff --git a/mm/internal.h b/mm/internal.h
index a3e19194079f..8dbb1335df88 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -11,6 +11,8 @@
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
#include <linux/tracepoint-defs.h>

struct folio_batch;
@@ -174,6 +176,52 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,

return min(ptep - start_ptep, max_nr);
}
+
+/**
+ * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
+ * @start_ptep: Page table pointer for the first entry.
+ * @max_nr: The maximum number of table entries to consider.
+ * @entry: Swap entry recovered from the first table entry.
+ *
+ * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs
+ * containing swap entries all with consecutive offsets and targeting the same
+ * swap type.
+ *
+ * max_nr must be at least one and must be limited by the caller so scanning
+ * cannot exceed a single page table.
+ *
+ * Return: the number of table entries in the batch.
+ */
+static inline int swap_pte_batch(pte_t *start_ptep, int max_nr,
+ swp_entry_t entry)
+{
+ const pte_t *end_ptep = start_ptep + max_nr;
+ unsigned long expected_offset = swp_offset(entry) + 1;
+ unsigned int expected_type = swp_type(entry);
+ pte_t *ptep = start_ptep + 1;
+
+ VM_WARN_ON(max_nr < 1);
+ VM_WARN_ON(non_swap_entry(entry));
+
+ while (ptep < end_ptep) {
+ pte_t pte = ptep_get(ptep);
+
+ if (pte_none(pte) || pte_present(pte))
+ break;
+
+ entry = pte_to_swp_entry(pte);
+
+ if (non_swap_entry(entry) ||
+ swp_type(entry) != expected_type ||
+ swp_offset(entry) != expected_offset)
+ break;
+
+ expected_offset++;
+ ptep++;
+ }
+
+ return ptep - start_ptep;
+}
#endif /* CONFIG_MMU */

void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
diff --git a/mm/madvise.c b/mm/madvise.c
index 44a498c94158..547dcd1f7a39 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -628,6 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
struct folio *folio;
int nr_swap = 0;
unsigned long next;
+ int nr, max_nr;

next = pmd_addr_end(addr, end);
if (pmd_trans_huge(*pmd))
@@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
return 0;
flush_tlb_batched_pending(mm);
arch_enter_lazy_mmu_mode();
- for (; addr != end; pte++, addr += PAGE_SIZE) {
+ for (; addr != end; pte += nr, addr += PAGE_SIZE * nr) {
+ nr = 1;
ptent = ptep_get(pte);

if (pte_none(ptent))
@@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,

entry = pte_to_swp_entry(ptent);
if (!non_swap_entry(entry)) {
- nr_swap--;
- free_swap_and_cache(entry);
- pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+ max_nr = (end - addr) / PAGE_SIZE;
+ nr = swap_pte_batch(pte, max_nr, entry);
+ nr_swap -= nr;
+ free_swap_and_cache_nr(entry, nr);
+ clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
} else if (is_hwpoison_entry(entry) ||
is_poisoned_swp_entry(entry)) {
pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..25c0ef1c7ff3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1629,12 +1629,13 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
folio_remove_rmap_pte(folio, page, vma);
folio_put(folio);
} else if (!non_swap_entry(entry)) {
- /* Genuine swap entry, hence a private anon page */
+ max_nr = (end - addr) / PAGE_SIZE;
+ nr = swap_pte_batch(pte, max_nr, entry);
+ /* Genuine swap entries, hence a private anon pages */
if (!should_zap_cows(details))
continue;
- rss[MM_SWAPENTS]--;
- if (unlikely(!free_swap_and_cache(entry)))
- print_bad_pte(vma, addr, ptent, NULL);
+ rss[MM_SWAPENTS] -= nr;
+ free_swap_and_cache_nr(entry, nr);
} else if (is_migration_entry(entry)) {
folio = pfn_swap_entry_folio(entry);
if (!should_zap_folio(details, folio))
@@ -1657,8 +1658,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
WARN_ON_ONCE(1);
}
- pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
- zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent);
+ clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
+ zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details, ptent);
} while (pte += nr, addr += PAGE_SIZE * nr, addr != end);

add_mm_rss_vec(mm, rss);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index df1de034f6d8..ee7e44cb40c5 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -130,7 +130,11 @@ static inline unsigned char swap_count(unsigned char ent)
/* Reclaim the swap entry if swap is getting full*/
#define TTRS_FULL 0x4

-/* returns 1 if swap entry is freed */
+/*
+ * returns number of pages in the folio that backs the swap entry. If positive,
+ * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
+ * folio was associated with the swap entry.
+ */
static int __try_to_reclaim_swap(struct swap_info_struct *si,
unsigned long offset, unsigned long flags)
{
@@ -155,6 +159,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
ret = folio_free_swap(folio);
folio_unlock(folio);
}
+ ret = ret ? folio_nr_pages(folio) : -folio_nr_pages(folio);
folio_put(folio);
return ret;
}
@@ -895,7 +900,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
swap_was_freed = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
spin_lock(&si->lock);
/* entry was freed successfully, try to use this again */
- if (swap_was_freed)
+ if (swap_was_freed > 0)
goto checks;
goto scan; /* check next one */
}
@@ -1572,32 +1577,63 @@ bool folio_free_swap(struct folio *folio)
return true;
}

-/*
- * Free the swap entry like above, but also try to
- * free the page cache entry if it is the last user.
- */
-int free_swap_and_cache(swp_entry_t entry)
+void free_swap_and_cache_nr(swp_entry_t entry, int nr)
{
- struct swap_info_struct *p;
- unsigned char count;
+ unsigned long end = swp_offset(entry) + nr;
+ unsigned int type = swp_type(entry);
+ struct swap_info_struct *si;
+ unsigned long offset;

if (non_swap_entry(entry))
- return 1;
+ return;

- p = get_swap_device(entry);
- if (p) {
- if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
- put_swap_device(p);
- return 0;
- }
+ si = get_swap_device(entry);
+ if (!si)
+ return;

- count = __swap_entry_free(p, entry);
- if (count == SWAP_HAS_CACHE)
- __try_to_reclaim_swap(p, swp_offset(entry),
+ if (WARN_ON(end > si->max))
+ goto out;
+
+ /*
+ * First free all entries in the range.
+ */
+ for (offset = swp_offset(entry); offset < end; offset++) {
+ if (!WARN_ON(data_race(!si->swap_map[offset])))
+ __swap_entry_free(si, swp_entry(type, offset));
+ }
+
+ /*
+ * Now go back over the range trying to reclaim the swap cache. This is
+ * more efficient for large folios because we will only try to reclaim
+ * the swap once per folio in the common case. If we do
+ * __swap_entry_free() and __try_to_reclaim_swap() in the same loop, the
+ * latter will get a reference and lock the folio for every individual
+ * page but will only succeed once the swap slot for every subpage is
+ * zero.
+ */
+ for (offset = swp_offset(entry); offset < end; offset += nr) {
+ nr = 1;
+ if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
+ /*
+ * Folios are always naturally aligned in swap so
+ * advance forward to the next boundary. Zero means no
+ * folio was found for the swap entry, so advance by 1
+ * in this case. Negative value means folio was found
+ * but could not be reclaimed. Here we can still advance
+ * to the next boundary.
+ */
+ nr = __try_to_reclaim_swap(si, offset,
TTRS_UNMAPPED | TTRS_FULL);
- put_swap_device(p);
+ if (nr == 0)
+ nr = 1;
+ else if (nr < 0)
+ nr = -nr;
+ nr = ALIGN(offset + 1, nr) - offset;
+ }
}
- return p != NULL;
+
+out:
+ put_swap_device(si);
}

#ifdef CONFIG_HIBERNATION
--
2.25.1