Message-ID: <CAGsJ_4xU1zYNvBObrKDOqRY7RE0R63d1VeTyRnKa8LPZt3rijw@mail.gmail.com>
Date: Tue, 9 Apr 2024 22:29:52 +1200
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: David Hildenbrand <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>, Gao Xiang <xiang@...nel.org>, 
	Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Chris Li <chrisl@...nel.org>, 
	Lance Yang <ioworker0@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

On Tue, Apr 9, 2024 at 9:55 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 09/04/2024 10:41, Barry Song wrote:
> > On Tue, Apr 9, 2024 at 9:24 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 09.04.24 11:22, Barry Song wrote:
> >>> On Tue, Apr 9, 2024 at 8:51 PM Barry Song <21cnbao@...il.com> wrote:
> >>>>
> >>>> On Tue, Apr 9, 2024 at 6:40 AM Ryan Roberts <ryan.roberts@....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@....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 :)

indeed. I wrote it in one minute.

I'm confident that the code can be written in a manner similar to
__pte_batch_clear_ignored. I was only proposing the approach,
not selling the code :-)

>
> >
> >> softdirty and uffd-wp as well at least initially for simplicity.
> >
> > yes for this.
> >
> > By the way, I wonder if you and Ryan have a moment to review swpin
> > refault patchset
> > v2 :-)
>
> It's on my todo list! I'm very keen to get as much large swap-out and swap-in
> support into v6.10 as we can. Hoping to get to it inthe next couple of days.
>
> >
> > [PATCH v2 0/5] large folios swap-in: handle refault cases first
> > https://lore.kernel.org/linux-mm/20240409082631.187483-1-21cnbao@gmail.com/
> >
> >
> >>
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >>
> >

Thanks
Barry
>

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