linux-kernel - Re: [PATCH RFC 23/39] mm/rmap: introduce folio_remove_rmap

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Date:   Tue, 5 Dec 2023 14:09:24 +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 RFC 23/39] mm/rmap: introduce
 folio_remove_rmap_[pte|ptes|pmd]()

>> +static __always_inline void __folio_remove_rmap(struct folio *folio,
>> +		struct page *page, unsigned int nr_pages,
>> +		struct vm_area_struct *vma, enum rmap_mode mode)
>> +{
>>   	atomic_t *mapped = &folio->_nr_pages_mapped;
>> -	int nr = 0, nr_pmdmapped = 0;
>> -	bool last;
>> +	int last, nr = 0, nr_pmdmapped = 0;
> 
> nit: you're being inconsistent across the functions with signed vs unsigned for
> page counts (e.g. nr, nr_pmdmapped) - see __folio_add_rmap(),
> __folio_add_file_rmap(), __folio_add_anon_rmap().
> 

Definitely.

> I suggest pick one and stick to it. Personally I'd go with signed int (since
> that's what all the counters in struct folio that we are manipulating are,
> underneath the atomic_t) then check that nr_pages > 0 in
> __folio_rmap_sanity_checks().

Can do, but note that the counters are signed to detect udnerflows. It 
doesn't make sense here to pass a negative number.

> 
>>   	enum node_stat_item idx;
>>   
>> -	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
>> -	VM_BUG_ON_PAGE(compound && !PageHead(page), page);
>> +	__folio_rmap_sanity_checks(folio, page, nr_pages, mode);
>>   
>>   	/* Is page being unmapped by PTE? Is this its last map to be removed? */
>> -	if (likely(!compound)) {
>> -		last = atomic_add_negative(-1, &page->_mapcount);
>> -		nr = last;
>> -		if (last && folio_test_large(folio)) {
>> -			nr = atomic_dec_return_relaxed(mapped);
>> -			nr = (nr < COMPOUND_MAPPED);
>> -		}
>> -	} else if (folio_test_pmd_mappable(folio)) {
>> -		/* That test is redundant: it's for safety or to optimize out */
>> +	if (likely(mode == RMAP_MODE_PTE)) {
>> +		do {
>> +			last = atomic_add_negative(-1, &page->_mapcount);
>> +			if (last && folio_test_large(folio)) {
>> +				last = atomic_dec_return_relaxed(mapped);
>> +				last = (last < COMPOUND_MAPPED);
>> +			}
>>   
>> +			if (last)
>> +				nr++;
>> +		} while (page++, --nr_pages > 0);
>> +	} else if (mode == RMAP_MODE_PMD) {
>>   		last = atomic_add_negative(-1, &folio->_entire_mapcount);
>>   		if (last) {
>>   			nr = atomic_sub_return_relaxed(COMPOUND_MAPPED, mapped);
>> @@ -1517,7 +1528,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>>   		 * is still mapped.
>>   		 */
>>   		if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
> 
> folio_test_pmd_mappable() -> folio_test_large()
> 
> Since you're converting this to support batch PTE removal, it might as well also
> support smaller-than-pmd too?

I remember that you have a patch for that, right? :)

> 
> I currently have a patch to do this same change in the multi-size THP series.
> 

Ah, yes, and that should go in first.


-- 
Cheers,

David / dhildenb