linux-kernel - Re: [PATCH v1 02/39] mm/rmap: introduce and use hugetlb_remove

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Date:   Mon, 11 Dec 2023 16:33:39 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Ryan Roberts <ryan.roberts@....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 02/39] mm/rmap: introduce and use hugetlb_remove_rmap()

On Mon, Dec 11, 2023 at 04:56:15PM +0100, David Hildenbrand wrote:
> hugetlb rmap handling differs quite a lot from "ordinary" rmap code.
> For example, hugetlb currently only supports entire mappings, and treats
> any mapping as mapped using a single "logical PTE". Let's move it out
> of the way so we can overhaul our "ordinary" rmap.
> implementation/interface.
> 
> Let's introduce and use hugetlb_remove_rmap() and remove the hugetlb
> code from page_remove_rmap(). This effectively removes one check on the
> small-folio path as well.
> 
> Note: all possible candidates that need care are page_remove_rmap() that
>       pass compound=true.
> 
> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

> +++ b/mm/rmap.c
> @@ -1482,13 +1482,6 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>  
>  	VM_BUG_ON_PAGE(compound && !PageHead(page), page);
>  
> -	/* Hugetlb pages are not counted in NR_*MAPPED */
> -	if (unlikely(folio_test_hugetlb(folio))) {
> -		/* hugetlb pages are always mapped with pmds */
> -		atomic_dec(&folio->_entire_mapcount);
> -		return;
> -	}

Maybe add
	VM_BUG_ON_FOLIO(folio_test_hugetlb(folio), folio);