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Date:   Thu, 12 May 2022 16:39:02 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yang Shi <shy828301@gmail.com>
Cc:     willy@infradead.org, songmuchun@bytedance.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: pvmw: check possible huge PMD map by
 transhuge_vma_suitable()
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On Thu, 12 May 2022 10:45:51 -0700 Yang Shi <shy828301@gmail.com> wrote:

> IIUC PVMW checks if the vma is possibly huge PMD mapped by
> transparent_hugepage_active() and "pvmw->nr_pages >= HPAGE_PMD_NR".
> 
> Actually pvmw->nr_pages is returned by compound_nr() or
> folio_nr_pages(), so the page should be THP as long as "pvmw->nr_pages
> >= HPAGE_PMD_NR".  And it is guaranteed THP is allocated for valid VMA
> in the first place.  But it may be not PMD mapped if the VMA is file
> VMA and it is not properly aligned.  The transhuge_vma_suitable()
> is used to do such check, so replace transparent_hugepage_active() to
> it, which is too heavy and overkilling.
> 
> ...
>
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -237,13 +237,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			spin_unlock(pvmw->ptl);
>  			pvmw->ptl = NULL;
>  		} else if (!pmd_present(pmde)) {
> +			unsigned long haddr = pvmw->address & HPAGE_PMD_MASK;

This hits

	#define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })

when CONFIG_TRANSPARENT_HUGEPAGE=n (x86_64 allnoconfig).