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Date: Tue, 9 Apr 2024 11:24:53 +0200
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Subject: Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched
 free_swap_and_cache()
To: Barry Song <21cnbao@gmail.com>, Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Huang Ying <ying.huang@intel.com>,
 Gao Xiang <xiang@kernel.org>, Yu Zhao <yuzhao@google.com>,
 Yang Shi <shy828301@gmail.com>, Michal Hocko <mhocko@suse.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>, Chris Li <chrisl@kernel.org>,
 Lance Yang <ioworker0@gmail.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
References: <20240408183946.2991168-1-ryan.roberts@arm.com>
 <20240408183946.2991168-3-ryan.roberts@arm.com>
 <CAGsJ_4xMaO8AWMGc4Od-FLWBhhT-u8f7QbR11VsqD0uqH3Kp6g@mail.gmail.com>
 <CAGsJ_4wpu2Nq0y3z7LYjPLJ1ZBi+BNRNKkyUPsXKDAg1POLjLA@mail.gmail.com>
From: David Hildenbrand <david@redhat.com>
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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