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Date: Wed, 31 Mar 2021 17:44:38 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Zi Yan <ziy@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Shuah Khan <shuah@kernel.org>, John Hubbard <jhubbard@nvidia.com>,
	Sandipan Das <sandipan@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Yang Shi <shy828301@gmail.com>,
	Mika Penttila <mika.penttila@nextfour.com>,
	David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v7 2/2] mm: huge_memory: debugfs for file-backed THP
 split.
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On Mon, Mar 29, 2021 at 11:39:32AM -0400, Zi Yan wrote:
> +	for (off_cur = off_start; off_cur < off_end;) {
> +		struct page *fpage = pagecache_get_page(mapping, off_cur,
> +						FGP_ENTRY | FGP_HEAD, 0);
> +
> +		if (xa_is_value(fpage) || !fpage) {
> +			off_cur += PAGE_SIZE;
> +			continue;
> +		}
> +
> +		if (!is_transparent_hugepage(fpage)) {
> +			off_cur += PAGE_SIZE;
> +			goto next;
> +		}
> +		total++;
> +		off_cur = fpage->index + thp_size(fpage);

That can't be right.  fpage->index is in units of pages and thp_size is
in units of bytes.  I wish C had a better type system.
I think you meant:

		off_cur = fpage->index + thp_nr_pages(fpage);

Also, I think this loop would read better as ...

	for (index = off_start; index < off_end; index += nr_pages) {
		struct page *fpage = pagecache_get_page(mapping, index,
						FGP_ENTRY | FGP_HEAD, 0);
		nr_pages = 1;
		if (xa_is_value(fpage) || !fpage)
			continue;
		if (!is_transparent_hugepage(fpage))
			goto next;
		total++;
		nr_pages = thp_nr_pages(fpage);
...