Re: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

From: Ryan Roberts
Date: Wed Apr 03 2024 - 03:17:17 EST

On 03/04/2024 04:57, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>
>> On 01/04/2024 06:52, Huang, Ying wrote:
>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>>
>>>> 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@xxxxxxx>
>>>> ---
>>>> 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 09c85c7bf9c2..8185939df1e8 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 f6f78198f000..5737236dc3ce 100644
>>>> --- a/include/linux/swap.h
>>>> +++ b/include/linux/swap.h
>>>> @@ -471,7 +471,7 @@ 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);
>>>> @@ -520,8 +520,9 @@ 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)
>>>> {
>>>> @@ -589,6 +590,11 @@ 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 8e11f7b2da21..eadb79c3a357 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;
>>>> @@ -189,6 +191,52 @@ 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 1f77a51baaac..070bedb4996e 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 36191a9c799c..9d844582ba38 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1631,12 +1631,13 @@ 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))
>>>> @@ -1659,8 +1660,8 @@ 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 0d44ee2b4f9c..cedfc82d37e5 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));
>>>
>>> I think that it's better to check the return value of
>>> __swap_entry_free() here. When the return value != SWAP_HAS_CACHE, we
>>> can try to reclaim all swap entries we have checked before, then restart
>>> the check with the new start.
>>
>> What's the benefit of your proposed aproach? I only see a drawback: if there are
>> large swap entries for which some pages have higher ref counts than others, we
>> will end up trying to reclaim (and fail) multiple times per folio. Whereas with
>> my current approach we only attempt reclaim once per folio.
>>
>> Do you see a specific bug with what I'm currently doing?
>
> No. Just want to find some opportunity to optimize. With my proposal,
> we can skip reclaim if "return value != SWAP_HAS_CACHE", for example, if
> the folio has been removed from the swap cache (fully reclaimed). But
> this has some drawbacks too, as you pointed out above.
>
> At least, we can check whether any return value of __swap_entry_free()
> == SWAP_HAS_CACHE, and only try to reclaim if so. This can optimize the
> most common cases.

Yes, good idea - I'll incorporate this in the next version. Thanks!

>
> [snip]
>
> --
> Best Regards,
> Huang, Ying