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

["Lance Yang" <lance.yang@linux.dev>]

April 3, 2024 at 8:30 AM, "Zi Yan" <ziy@nvidia.com> wrote:



> 
> On 27 Mar 2024, at 10:45, Ryan Roberts 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 | 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
> > 
> >  +
> > 
> 
> Would the code below be better?
> 
> for (i = 0; i < nr; i++, ptep++, addr += PAGE_SIZE)
> 

FWIW

for (; nr-- > 0; ptep++, addr += PAGE_SIZE)
  pte_clear_not_present_full(mm, addr, ptep, full);

Thanks,
Lance


>  pte_clear_not_present_full(mm, addr, ptep, full);
> 
> --
> 
> Best Regards,
> 
> Yan, Zi
>