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

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Message-ID: <B74255A8-0103-4888-830B-0ADC60947E8E@nvidia.com>
Date:   Wed, 31 Mar 2021 13:00:08 -0400
From:   "Zi Yan" <ziy@...dia.com>
To:     "Matthew Wilcox" <willy@...radead.org>
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 31 Mar 2021, at 12:44, Matthew Wilcox wrote:

> 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);
> ...

Thanks for catching this! I mixed this with looping through VMA, which
is in units of bytes. I will fix this and use your suggested loop code.

—
Best Regards,
Yan Zi

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