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Date:   Mon, 17 Jul 2023 15:19:41 +0200
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To:     Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        Luis Chamberlain <mcgrof@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20230714160407.4142030-1-ryan.roberts@arm.com>
 <20230714161733.4144503-1-ryan.roberts@arm.com>
 <d85c0f04-1792-2a0f-d0be-7fffc7604797@redhat.com>
 <fccc8f3e-126f-baab-a14d-387018e5b8b7@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for
 folio_add_new_anon_rmap()
In-Reply-To: <fccc8f3e-126f-baab-a14d-387018e5b8b7@arm.com>
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On 17.07.23 15:13, Ryan Roberts wrote:
> On 17/07/2023 14:00, David Hildenbrand wrote:
>> On 14.07.23 18:17, Ryan Roberts wrote:
>>> In preparation for FLEXIBLE_THP support, improve
>>> folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
>>> passed to it. In this case, all contained pages are accounted using the
>>> order-0 folio (or base page) scheme.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> Reviewed-by: Yu Zhao <yuzhao@google.com>
>>> Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
>>> ---
>>>    mm/rmap.c | 28 +++++++++++++++++++++-------
>>>    1 file changed, 21 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 0c0d8857dfce..f293d072368a 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1278,31 +1278,45 @@ void page_add_anon_rmap(struct page *page, struct
>>> vm_area_struct *vma,
>>>     * This means the inc-and-test can be bypassed.
>>>     * The folio does not have to be locked.
>>>     *
>>> - * If the folio is large, it is accounted as a THP.  As the folio
>>> + * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
>>>     * is new, it's assumed to be mapped exclusively by a single process.
>>>     */
>>>    void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>>>            unsigned long address)
>>>    {
>>> -    int nr;
>>> +    int nr = folio_nr_pages(folio);
>>>
>>> -    VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
>>> +    VM_BUG_ON_VMA(address < vma->vm_start ||
>>> +            address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
>>>        __folio_set_swapbacked(folio);
>>>
>>> -    if (likely(!folio_test_pmd_mappable(folio))) {
>>> +    if (!folio_test_large(folio)) {
>>
>> Why remove the "likely" here? The patch itself does not change anything about
>> that condition.
> 
> Good question; I'm not sure why. Will have to put it down to bad copy/paste
> fixup. Will put it back in the next version.
> 
>>
>>>            /* increment count (starts at -1) */
>>>            atomic_set(&folio->_mapcount, 0);
>>> -        nr = 1;
>>> +        __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>>> +    } else if (!folio_test_pmd_mappable(folio)) {
>>> +        int i;
>>> +
>>> +        for (i = 0; i < nr; i++) {
>>> +            struct page *page = folio_page(folio, i);
>>> +
>>> +            /* increment count (starts at -1) */
>>> +            atomic_set(&page->_mapcount, 0);
>>> +            __page_set_anon_rmap(folio, page, vma,
>>> +                    address + (i << PAGE_SHIFT), 1);
>>> +        }
>>> +
>>> +        /* increment count (starts at 0) */
>>
>> That comment is a bit misleading. We're not talking about a mapcount as in the
>> other cases here.
> 
> Correct, I'm talking about _nr_pages_mapped, which starts 0, not -1 like
> _mapcount. The comment was intended to be in the style used in other similar
> places in rmap.c. I could change it to: "_nr_pages_mapped is 0-based, so set it
> to the number of pages in the folio" or remove it entirely? What do you prefer?
> 

We only have to comment what's weird, not what's normal.

IOW, we also didn't have such a comment in the existing code when doing 
atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);


What might make sense here is a simple

"All pages of the folio are PTE-mapped."

-- 
Cheers,

David / dhildenb