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

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Message-ID: <6ceb7a49-0039-4faf-a069-7d47d74790f6@arm.com>
Date: Tue, 19 Dec 2023 08:40:56 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 18/12/2023 17:06, David Hildenbrand wrote:
> 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.

Well it doesn't really mean much to me in this new context, so I would reword if
there is still something you need to convey to the reader, else just remove.

> 
>>
>>> +    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.
> 

Well it's an easy way to detect overflow? But I know what you mean. There are
lots of other APIs that accept signed/unsigned 32/64 bits; It's a mess. It would
be a tiny step in the right direction if a series could at least be consistent
with itself though, IMHO. :)