linux-kernel - Re: [PATCH RFC v2] ksm: Assist buddy allocator to assemble 1-order pages

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Date:   Mon, 15 Oct 2018 11:56:08 +0000
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
CC:     "hughd@...gle.com" <hughd@...gle.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "rppt@...ux.vnet.ibm.com" <rppt@...ux.vnet.ibm.com>,
        "imbrenda@...ux.vnet.ibm.com" <imbrenda@...ux.vnet.ibm.com>,
        "corbet@....net" <corbet@....net>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "jia.he@...-semitech.com" <jia.he@...-semitech.com>,
        "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        "jiang.biao2@....com.cn" <jiang.biao2@....com.cn>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2] ksm: Assist buddy allocator to assemble 1-order
 pages

On 15.10.2018 13:38, Claudio Imbrenda wrote:
> I don't have objections to this patch, but I wonder how much impact it
> would have. Have you performed any tests? does it really have such a big
> impact on the availability of order-1 page blocks? 

I have no synthetic tests on compaction, so this patch is RFC. Maybe you
suggest something? In my test machine I added debug patch on top of this,
which adds a counter of such tree_page preferred pages, and the counter
increments as well. Order-1 page is just a brick of a bigger order pages,
so this patch cares about them.

> 
> On Mon, 15 Oct 2018 12:33:36 +0300
> Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> 
>> v2: Style improvements
>>
>> try_to_merge_two_pages() merges two pages, one of them
>> is a page of currently scanned mm, the second is a page
>> with identical hash from unstable tree. Currently, we
>> merge the page from unstable tree into the first one,
>> and then free it.
>>
>> The idea of this patch is to prefer freeing that page
>> of them, which has a free neighbour (i.e., neighbour
>> with zero page_count()). This allows buddy allocator
>> to assemble at least 1-order set from the freed page
>> and its neighbour; this is a kind of cheep passive
>> compaction.
>>
>> AFAIK, 1-order pages set consists of pages with PFNs
>> [2n, 2n+1] (odd, even), so the neighbour's pfn is
>> calculated via XOR with 1. We check the result pfn
>> is valid and its page_count(), and prefer merging
>> into @tree_page if neighbour's usage count is zero.
>>
>> There a is small difference with current behavior
>> in case of error path. In case of the second
>> try_to_merge_with_ksm_page() is failed, we return
>> from try_to_merge_two_pages() with @tree_page
>> removed from unstable tree. It does not seem to matter,
>> but if we do not want a change at all, it's not
>> a problem to move remove_rmap_item_from_tree() from
>> try_to_merge_with_ksm_page() to its callers.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
>> ---
>>  mm/ksm.c |   16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>>
>> diff --git a/mm/ksm.c b/mm/ksm.c
>> index 5b0894b45ee5..005508c86d0a 100644
>> --- a/mm/ksm.c
>> +++ b/mm/ksm.c
>> @@ -1321,6 +1321,22 @@ static struct page
>> *try_to_merge_two_pages(struct rmap_item *rmap_item, {
>>  	int err;
>>
>> +	if (IS_ENABLED(CONFIG_COMPACTION)) {
>> +		unsigned long pfn;
>> +
>> +		/*
>> +		 * Find neighbour of @page containing 1-order pair
>> +		 * in buddy-allocator and check whether it is free.
>> +		 * If it is so, try to use @tree_page as ksm page
>> +		 * and to free @page.
>> +		 */
>> +		pfn = page_to_pfn(page) ^ 1;
>> +		if (pfn_valid(pfn) && page_count(pfn_to_page(pfn))
>> == 0) {
>> +			swap(rmap_item, tree_rmap_item);
>> +			swap(page, tree_page);
>> +		}
>> +	}
>> +
>>  	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
>>  	if (!err) {
>>  		err = try_to_merge_with_ksm_page(tree_rmap_item,
>>
>