linux-kernel - Re: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and

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Date: Fri, 5 Apr 2024 11:24:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>
Cc: 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>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>, Lance Yang <ioworker0@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched
 free_swap_and_cache()

On 03.04.24 09:21, Ryan Roberts wrote:
> 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@....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)
>> 	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.

Yes please. I remember Willy found that variant to be most micro-optimized.

-- 
Cheers,

David / dhildenb