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

[Ryan Roberts]

On 03/04/2024 01:30, Zi Yan 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@xxxxxxx>
>> ---
>>  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)
> 	pte_clear_not_present_full(mm, addr, ptep, full);

I certainly agree that this is cleaner and more standard. But I'm copying the
pattern used by the other batch helpers. I believe this pattern was first done
by Willy for set_ptes(), then continued by DavidH for wrprotect_ptes() and
clear_full_ptes().

I guess the benefit is that ptep and addr are only incremented if we are going
around the loop again. I'd rather continue to be consistent with those other
helpers.


> 
> --
> Best Regards,
> Yan, Zi