Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 9 Apr 2024 12:42:39 +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 11:55, Ryan Roberts wrote: > On 09/04/2024 10:41, Barry Song wrote: >> On Tue, Apr 9, 2024 at 9:24 PM David Hildenbrand <david@redhat.com> wrote: >>> >>> 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. >> >> I am not quite sure I definitely need exclusive bits to match. i can either >> drop my 3/5 or ignore the exclusive bit as below (if anyone is not shared, >> swpin won't reuse the large folio, but it can still entirely map it read-only): >> >> diff --git a/mm/internal.h b/mm/internal.h >> index cae39c372bfc..5726e729c9ee 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -253,10 +253,22 @@ static inline int swap_pte_batch(pte_t >> *start_ptep, int max_nr, pte_t pte, >> *any_shared |= !pte_swp_exclusive(pte); >> >> while (ptep < end_ptep) { >> + pte_t ignore_exclusive_pte; >> + pte_t ignore_exclusive_expected_pte; >> pte = ptep_get(ptep); >> >> - if (!pte_same(pte, expected_pte)) >> - break; >> + if (any_shared) { >> + ignore_exclusive_pte = pte; >> + ignore_exclusive_expected_pte = expected_pte; >> + ignore_exclusive_pte = >> pte_swp_clear_exclusive(ignore_exclusive_pte); >> + ignore_exclusive_expected_pte = >> pte_swp_clear_exclusive(expected_pte); >> + >> + if (!pte_same(ignore_exclusive_pte, >> ignore_exclusive_expected_pte)) >> + break; >> + } else { >> + if (!pte_same(pte, expected_pte)) >> + break; >> + } >> >> if (any_shared) >> *any_shared |= !pte_swp_exclusive(pte); > > I'll leave David to comment on this proposal; I'm not sure I understand all the > details. The code change does look a bit "busy" though - sometimes that can be > an indicator :)
I'd prefer to keep it simple initially. Devil is in the detail for the refault case: in the past, dropping an exclusive flag could have been problematic with some O_DIRECT workloads that were not using FOLL_PIN. IIUC, some still remain.
More details can be had from https://lore.kernel.org/all/20230113171026.582290-1-david@redhat.com/
It might all change a bit if I manage to get folio_test_anon_exclusive() flying. The current plan is that all individual swp PTEs would still have pte_swp_exclusive() set, and we would *not* clear the folio_test_anon_exclusive() flag while the folio is still in the swapcache [which we do today to make fork() handling easier].
That will make refault significantly easier to handle regarding exclusivity handling with large folios.
-- Cheers,
David / dhildenb
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