linux-kernel - Re: [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

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Message-ID: <970e6480-5ba0-4500-85a6-f7ec6db2f005@redhat.com>
Date: Wed, 18 Jun 2025 19:29:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 baohua@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] khugepaged: Optimize
 __collapse_huge_page_copy_succeeded() for large folios by PTE batching

On 18.06.25 19:26, Lorenzo Stoakes wrote:
> On Wed, Jun 18, 2025 at 03:56:07PM +0530, Dev Jain wrote:
>> Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
>>
>> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
>> Then, calling ptep_clear() for every pte will cause a TLB flush for every
>> contpte block. Instead, clear_full_ptes() does a
>> contpte_try_unfold_partial() which will flush the TLB only for the (if any)
>> starting and ending contpte block, if they partially overlap with the range
>> khugepaged is looking at.
>>
>> For all arches, there should be a benefit due to batching atomic operations
>> on mapcounts due to folio_remove_rmap_ptes().
>>
>> No issues were observed with mm-selftests.
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>   mm/khugepaged.c | 31 +++++++++++++++++++++++--------
>>   1 file changed, 23 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d45d08b521f6..649ccb2670f8 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -700,12 +700,14 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>>   						spinlock_t *ptl,
>>   						struct list_head *compound_pagelist)
>>   {
>> +	unsigned long end = address + HPAGE_PMD_SIZE;
> 
> I assume we always enter here with aligned address...
> 
>>   	struct folio *src, *tmp;
>> -	pte_t *_pte;
>> +	pte_t *_pte = pte;
>>   	pte_t pteval;
>> +	int nr_ptes;
>>
>> -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
>> -	     _pte++, address += PAGE_SIZE) {
>> +	do {
>> +		nr_ptes = 1;
>>   		pteval = ptep_get(_pte);
>>   		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>>   			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
>> @@ -719,23 +721,36 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>>   				ksm_might_unmap_zero_page(vma->vm_mm, pteval);
>>   			}
>>   		} else {
> 
> Existing code but hate this level of indentation.
> 
> The code before was (barely) sort of ok-ish, but now it's realyl out of hand.
> 
> On the other hand, I look at __collapse_huge_page_isolate() and want to cry so I
> guess this maybe is something that needs addressing outside of this patch.
> 
> 
>> +			const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +			int max_nr_ptes;
>> +			bool is_large;
>> +
>>   			struct page *src_page = pte_page(pteval);
>>
>>   			src = page_folio(src_page);
>> -			if (!folio_test_large(src))
>> +			is_large = folio_test_large(src);
>> +			if (!is_large)
>>   				release_pte_folio(src);
> 
> Hm, in this case right, release_pte_folio() does a folio_unlock().
> 
> Where does a large folio get unlocked?

Through the "compound_pagelist" below. ... this code is so ugly.

"large_folio_list" ...

-- 
Cheers,

David / dhildenb