linux-kernel - Re: [PATCH -mm] mm: Clear to access sub-page last when clearing huge page

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Date:   Tue, 08 Aug 2017 07:05:03 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nadia Yvette Chambers <nyc@...omorphy.com>,
        Michal Hocko <mhocko@...e.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>, Shaohua Li <shli@...com>
Subject: Re: [PATCH -mm] mm: Clear to access sub-page last when clearing huge page

Christopher Lameter <cl@...ux.com> writes:

> On Mon, 7 Aug 2017, Huang, Ying wrote:
>
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4374,9 +4374,31 @@ void clear_huge_page(struct page *page,
>>  	}
>>
>>  	might_sleep();
>> -	for (i = 0; i < pages_per_huge_page; i++) {
>> +	VM_BUG_ON(clamp(addr_hint, addr, addr +
>> +			(pages_per_huge_page << PAGE_SHIFT)) != addr_hint);
>> +	n = (addr_hint - addr) / PAGE_SIZE;
>> +	if (2 * n <= pages_per_huge_page) {
>> +		base = 0;
>> +		l = n;
>> +		for (i = pages_per_huge_page - 1; i >= 2 * n; i--) {
>> +			cond_resched();
>> +			clear_user_highpage(page + i, addr + i * PAGE_SIZE);
>> +		}
>
> I really like the idea behind the patch but this is not clearing from last
> to first byte of the huge page.
>
> What seems to be happening here is clearing from the last page to the
> first page and I would think that within each page the clearing is from
> first byte to last byte. Maybe more gains can be had by really clearing
> from last to first byte of the huge page instead of this jumping over 4k
> addresses?

Yes.  That is a good idea.  I will experiment it via changing the
direction to clear in clear_user_highpage().

Best Regards,
Huang, Ying