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

Ryan Roberts
2024-03-20T11:10:32Z

Hi David,

I hate to chase, but since you provided feedback on a couple of the other
patches in the series, I wondered if you missed this one? It's the one that does
the batching of free_swap_and_cache(), which you suggested in order to prevent
needlessly taking folio locks and refs.

If you have any feedback, it would be appreciated, otherwise I'm planning to
repost as-is next week (nobody else has posted comments against this patch
either) as part of the updated series.

Thanks,
Ryan

On 11/03/2024 15:00, Ryan Roberts wrote:
> 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