linux-kernel - Re: [PATCH v1 13/39] mm/rmap: factor out adding folio mappings into __folio_add

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Date: Mon, 18 Dec 2023 18:06:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Hugh Dickins <hughd@...gle.com>, Yin Fengwei <fengwei.yin@...el.com>,
 Mike Kravetz <mike.kravetz@...cle.com>, Muchun Song <muchun.song@...ux.dev>,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 13/39] mm/rmap: factor out adding folio mappings into
 __folio_add_rmap()

On 18.12.23 17:07, Ryan Roberts wrote:
> On 11/12/2023 15:56, David Hildenbrand wrote:
>> Let's factor it out to prepare for reuse as we convert
>> page_add_anon_rmap() to folio_add_anon_rmap_[pte|ptes|pmd]().
>>
>> Make the compiler always special-case on the granularity by using
>> __always_inline.
>>
>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   mm/rmap.c | 81 ++++++++++++++++++++++++++++++-------------------------
>>   1 file changed, 45 insertions(+), 36 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2ff2f11275e5..c5761986a411 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1157,6 +1157,49 @@ int folio_total_mapcount(struct folio *folio)
>>   	return mapcount;
>>   }
>>   
>> +static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
>> +		struct page *page, int nr_pages, enum rmap_mode mode,
>> +		unsigned int *nr_pmdmapped)
>> +{
>> +	atomic_t *mapped = &folio->_nr_pages_mapped;
>> +	int first, nr = 0;
>> +
>> +	__folio_rmap_sanity_checks(folio, page, nr_pages, mode);
>> +
>> +	/* Is page being mapped by PTE? Is this its first map to be added? */
> 
> I suspect this comment is left over from the old version? It sounds a bit odd in
> its new context.

In this patch, I'm just moving the code, so it would have to be dropped 
in a previous patch.

I'm happy to drop all these comments in previous patches.

> 
>> +	switch (mode) {
>> +	case RMAP_MODE_PTE:
>> +		do {
>> +			first = atomic_inc_and_test(&page->_mapcount);
>> +			if (first && folio_test_large(folio)) {
>> +				first = atomic_inc_return_relaxed(mapped);
>> +				first = (first < COMPOUND_MAPPED);
>> +			}
>> +
>> +			if (first)
>> +				nr++;
>> +		} while (page++, --nr_pages > 0);
>> +		break;
>> +	case RMAP_MODE_PMD:
>> +		first = atomic_inc_and_test(&folio->_entire_mapcount);
>> +		if (first) {
>> +			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
>> +			if (likely(nr < COMPOUND_MAPPED + COMPOUND_MAPPED)) {
>> +				*nr_pmdmapped = folio_nr_pages(folio);
>> +				nr = *nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
>> +				/* Raced ahead of a remove and another add? */
>> +				if (unlikely(nr < 0))
>> +					nr = 0;
>> +			} else {
>> +				/* Raced ahead of a remove of COMPOUND_MAPPED */
>> +				nr = 0;
>> +			}
>> +		}
>> +		break;
>> +	}
>> +	return nr;
>> +}
>> +
>>   /**
>>    * folio_move_anon_rmap - move a folio to our anon_vma
>>    * @folio:	The folio to move to our anon_vma
>> @@ -1380,45 +1423,11 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
>>   		struct page *page, int nr_pages, struct vm_area_struct *vma,
>>   		enum rmap_mode mode)
>>   {
>> -	atomic_t *mapped = &folio->_nr_pages_mapped;
>> -	unsigned int nr_pmdmapped = 0, first;
>> -	int nr = 0;
>> +	unsigned int nr, nr_pmdmapped = 0;
> 
> You're still being inconsistent with signed/unsigned here. Is there a reason
> these can't be signed like nr_pages in the interface?

I can turn them into signed values.

Personally, I think it's misleading to use "signed" for values that have 
absolutely no meaning for negative meaning. But sure, we can be 
consistent, at least in rmap code.

-- 
Cheers,

David / dhildenb