Messages in this thread | | | Date | Tue, 9 Apr 2024 11:24:53 +0200 | Subject | Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache() | From | David Hildenbrand <> |
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On 09.04.24 11:22, Barry Song wrote: > On Tue, Apr 9, 2024 at 8:51 PM Barry Song <21cnbao@gmail.com> wrote: >> >> On Tue, Apr 9, 2024 at 6:40 AM Ryan Roberts <ryan.roberts@arm.com> 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 | 29 ++++++++++++ >>> include/linux/swap.h | 12 +++-- >>> mm/internal.h | 63 ++++++++++++++++++++++++++ >>> mm/madvise.c | 12 +++-- >>> mm/memory.c | 13 +++--- >>> mm/swapfile.c | 97 +++++++++++++++++++++++++++++++++-------- >>> 6 files changed, 195 insertions(+), 31 deletions(-) >>> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>> index a3fc8150b047..75096025fe52 100644 >>> --- a/include/linux/pgtable.h >>> +++ b/include/linux/pgtable.h >>> @@ -708,6 +708,35 @@ 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 multiple not present PTEs which are >>> + * consecutive in the pgtable. >>> + * @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 3bdc8693b54f..de68705624b0 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 +191,67 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr, >>> >>> return min(ptep - start_ptep, max_nr); >>> } >>> + >>> +/** >>> + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte. >>> + * @pte: The initial pte state; is_swap_pte(pte) must be true. >>> + * >>> + * Increments the swap offset, while maintaining all other fields, including >>> + * swap type, and any swp pte bits. The resulting pte is returned. >>> + */ >>> +static inline pte_t pte_next_swp_offset(pte_t pte) >>> +{ >>> + swp_entry_t entry = pte_to_swp_entry(pte); >>> + pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry), >>> + swp_offset(entry) + 1)); >>> + >>> + if (pte_swp_soft_dirty(pte)) >>> + new = pte_swp_mksoft_dirty(new); >>> + if (pte_swp_exclusive(pte)) >>> + new = pte_swp_mkexclusive(new); >>> + if (pte_swp_uffd_wp(pte)) >>> + new = pte_swp_mkuffd_wp(new); >> >> I don't quite understand this. If this page table entry is exclusive, >> will its subsequent page table entry also be exclusive without >> question? >> in try_to_unmap_one, exclusive is per-subpage but not per-folio: >> >> anon_exclusive = folio_test_anon(folio) && >> PageAnonExclusive(subpage); >> >> same questions also for diry, wp etc. > > Sorry for the noise. you are right. based on your new version, I think I should > entirely drop: > > [PATCH v2 3/5] mm: swap_pte_batch: add an output argument to reture if > all swap entries are exclusive
Yes. If we ever want to ignore some bits, we should likely add flags to change the behavior, like for folio_pte_batch().
For swapin, you really want the exclusive bits to match, though. softdirty and uffd-wp as well at least initially for simplicity.
-- Cheers,
David / dhildenb
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