linux-kernel - Re: [PATCH v7 2/2] mm: huge_memory: debugfs for file-backed THP split.

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Date:   Wed, 31 Mar 2021 17:44:38 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Mika Penttila <mika.penttila@...tfour.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v7 2/2] mm: huge_memory: debugfs for file-backed THP
 split.

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);
...