Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <d85c0f04-1792-2a0f-d0be-7fffc7604797@redhat.com>
Date:   Mon, 17 Jul 2023 15:00:07 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.12.0
Subject: Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for
 folio_add_new_anon_rmap()
Content-Language: en-US
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>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230714161733.4144503-1-ryan.roberts@arm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.

>   		/* 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.

> +		atomic_set(&folio->_nr_pages_mapped, nr);
>   	} else {
>   		/* increment count (starts at -1) */
>   		atomic_set(&folio->_entire_mapcount, 0);
>   		atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
> -		nr = folio_nr_pages(folio);
> +		__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>   		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
>   	}
> 

Apart from that, LGTM.

-- 
Cheers,

David / dhildenb