Message-ID: <43350F6E-E56D-41C8-8441-A96D83B7D7C1@nvidia.com>
Date:   Fri, 13 Oct 2023 10:51:24 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene

On 12 Oct 2023, at 20:06, Zi Yan wrote:

> On 10 Oct 2023, at 17:12, Johannes Weiner wrote:
>
>> Hello!
>>
>> On Mon, Oct 02, 2023 at 10:26:44PM -0400, Zi Yan wrote:
>>> On 27 Sep 2023, at 22:51, Zi Yan wrote:
>>> I attached my revised patch 2 and 3 (with all the suggestions above).
>>
>> Thanks! It took me a bit to read through them. It's a tricky codebase!
>>
>> Some comments below.
>>
>>> From 1c8f99cff5f469ee89adc33e9c9499254cad13f2 Mon Sep 17 00:00:00 2001
>>> From: Zi Yan <ziy@...dia.com>
>>> Date: Mon, 25 Sep 2023 16:27:14 -0400
>>> Subject: [PATCH v2 1/2] mm: set migratetype after free pages are moved between
>>>  free lists.
>>>
>>> This avoids changing migratetype after move_freepages() or
>>> move_freepages_block(), which is error prone. It also prepares for upcoming
>>> changes to fix move_freepages() not moving free pages partially in the
>>> range.
>>>
>>> Signed-off-by: Zi Yan <ziy@...dia.com>
>>
>> This is great and indeed makes the callsites much simpler. Thanks,
>> I'll fold this into the series.
>>
>>> @@ -1597,9 +1615,29 @@ static int move_freepages(struct zone *zone, unsigned long start_pfn,
>>>  			  unsigned long end_pfn, int old_mt, int new_mt)
>>>  {
>>>  	struct page *page;
>>> -	unsigned long pfn;
>>> +	unsigned long pfn, pfn2;
>>>  	unsigned int order;
>>>  	int pages_moved = 0;
>>> +	unsigned long mt_changed_pfn = start_pfn - pageblock_nr_pages;
>>> +	unsigned long new_start_pfn = get_freepage_start_pfn(start_pfn);
>>> +
>>> +	/* split at start_pfn if it is in the middle of a free page */
>>> +	if (new_start_pfn != start_pfn && PageBuddy(pfn_to_page(new_start_pfn))) {
>>> +		struct page *new_page = pfn_to_page(new_start_pfn);
>>> +		int new_page_order = buddy_order(new_page);
>>
>> get_freepage_start_pfn() returns start_pfn if it didn't find a large
>> buddy, so the buddy check shouldn't be necessary, right?
>>
>>> +		if (new_start_pfn + (1 << new_page_order) > start_pfn) {
>>
>> This *should* be implied according to the comments on
>> get_freepage_start_pfn(), but it currently isn't. Doing so would help
>> here, and seemingly also in alloc_contig_range().
>>
>> How about this version of get_freepage_start_pfn()?
>>
>> /*
>>  * Scan the range before this pfn for a buddy that straddles it
>>  */
>> static unsigned long find_straddling_buddy(unsigned long start_pfn)
>> {
>> 	int order = 0;
>> 	struct page *page;
>> 	unsigned long pfn = start_pfn;
>>
>> 	while (!PageBuddy(page = pfn_to_page(pfn))) {
>> 		/* Nothing found */
>> 		if (++order > MAX_ORDER)
>> 			return start_pfn;
>> 		pfn &= ~0UL << order;
>> 	}
>>
>> 	/*
>> 	 * Found a preceding buddy, but does it straddle?
>> 	 */
>> 	if (pfn + (1 << buddy_order(page)) > start_pfn)
>> 		return pfn;
>>
>> 	/* Nothing found */
>> 	return start_pfn;
>> }
>>
>>> @@ -1614,10 +1652,43 @@ static int move_freepages(struct zone *zone, unsigned long start_pfn,
>>>
>>>  		order = buddy_order(page);
>>>  		move_to_free_list(page, zone, order, old_mt, new_mt);
>>> +		/*
>>> +		 * set page migratetype 1) only after we move all free pages in
>>> +		 * one pageblock and 2) for all pageblocks within the page.
>>> +		 *
>>> +		 * for 1), since move_to_free_list() checks page migratetype with
>>> +		 * old_mt and changing one page migratetype affects all pages
>>> +		 * within the same pageblock, if we are moving more than
>>> +		 * one free pages in the same pageblock, setting migratetype
>>> +		 * right after first move_to_free_list() triggers the warning
>>> +		 * in the following move_to_free_list().
>>> +		 *
>>> +		 * for 2), when a free page order is greater than pageblock_order,
>>> +		 * all pageblocks within the free page need to be changed after
>>> +		 * move_to_free_list().
>>
>> I think this can be somewhat simplified.
>>
>> There are two assumptions we can make. Buddies always consist of 2^n
>> pages. And buddies and pageblocks are naturally aligned. This means
>> that if this pageblock has the start of a buddy that straddles into
>> the next pageblock(s), it must be the first page in the block. That in
>> turn means we can move the handling before the loop.
>>
>> If we split first, it also makes the loop a little simpler because we
>> know that any buddies that start inside this block cannot extend
>> beyond it (due to the alignment). The loop how it was originally
>> written can remain untouched.
>>
>>> +		 */
>>> +		if (pfn + (1 << order) > pageblock_end_pfn(pfn)) {
>>> +			for (pfn2 = pfn;
>>> +			     pfn2 < min_t(unsigned long,
>>> +					  pfn + (1 << order),
>>> +					  end_pfn + 1);
>>> +			     pfn2 += pageblock_nr_pages) {
>>> +				set_pageblock_migratetype(pfn_to_page(pfn2),
>>> +							  new_mt);
>>> +				mt_changed_pfn = pfn2;
>>
>> Hm, this seems to assume that start_pfn to end_pfn can be more than
>> one block. Why is that? This function is only used on single blocks.
>
> You are right. I made unnecessary assumptions when I wrote the code.
>
>>
>>> +			}
>>> +			/* split the free page if it goes beyond the specified range */
>>> +			if (pfn + (1 << order) > (end_pfn + 1))
>>> +				split_free_page(page, order, end_pfn + 1 - pfn);
>>> +		}
>>>  		pfn += 1 << order;
>>>  		pages_moved += 1 << order;
>>>  	}
>>> -	set_pageblock_migratetype(pfn_to_page(start_pfn), new_mt);
>>> +	/* set migratetype for the remaining pageblocks */
>>> +	for (pfn2 = mt_changed_pfn + pageblock_nr_pages;
>>> +	     pfn2 <= end_pfn;
>>> +	     pfn2 += pageblock_nr_pages)
>>> +		set_pageblock_migratetype(pfn_to_page(pfn2), new_mt);
>>
>> If I rework the code on the above, I'm arriving at the following:
>>
>> static int move_freepages(struct zone *zone, unsigned long start_pfn,
>> 			  unsigned long end_pfn, int old_mt, int new_mt)
>> {
>> 	struct page *start_page = pfn_to_page(start_pfn);
>> 	int pages_moved = 0;
>> 	unsigned long pfn;
>>
>> 	VM_WARN_ON(start_pfn & (pageblock_nr_pages - 1));
>> 	VM_WARN_ON(start_pfn + pageblock_nr_pages - 1 != end_pfn);
>>
>> 	/*
>> 	 * A free page may be comprised of 2^n blocks, which means our
>> 	 * block of interest could be head or tail in such a page.
>> 	 *
>> 	 * If we're a tail, update the type of our block, then split
>> 	 * the page into pageblocks. The splitting will do the leg
>> 	 * work of sorting the blocks into the right freelists.
>> 	 *
>> 	 * If we're a head, split the page into pageblocks first. This
>> 	 * ensures the migratetypes still match up during the freelist
>> 	 * removal. Then do the regular scan for buddies in the block
>> 	 * of interest, which will handle the rest.
>> 	 *
>> 	 * In theory, we could try to preserve 2^1 and larger blocks
>> 	 * that lie outside our range. In practice, MAX_ORDER is
>> 	 * usually one or two pageblocks anyway, so don't bother.
>> 	 *
>> 	 * Note that this only applies to page isolation, which calls
>> 	 * this on random blocks in the pfn range! When we move stuff
>> 	 * from inside the page allocator, the pages are coming off
>> 	 * the freelist (can't be tail) and multi-block pages are
>> 	 * handled directly in the stealing code (can't be a head).
>> 	 */
>>
>> 	/* We're a tail */
>> 	pfn = find_straddling_buddy(start_pfn);
>> 	if (pfn != start_pfn) {
>> 		struct page *free_page = pfn_to_page(pfn);
>>
>> 		set_pageblock_migratetype(start_page, new_mt);
>> 		split_free_page(free_page, buddy_order(free_page),
>> 				pageblock_nr_pages);
>> 		return pageblock_nr_pages;
>> 	}
>>
>> 	/* We're a head */
>> 	if (PageBuddy(start_page) && buddy_order(start_page) > pageblock_order)
>> 		split_free_page(start_page, buddy_order(start_page),
>> 				pageblock_nr_pages);
>
> This actually can be:
>
> /* We're a head */
> if (PageBuddy(start_page) && buddy_order(start_page) > pageblock_order) {
>         set_pageblock_migratetype(start_page, new_mt);
>         split_free_page(start_page, buddy_order(start_page),
>                         pageblock_nr_pages);
>         return pageblock_nr_pages;
> }
>
>
>>
>> 	/* Move buddies within the block */
>> 	while (pfn <= end_pfn) {
>> 		struct page *page = pfn_to_page(pfn);
>> 		int order, nr_pages;
>>
>> 		if (!PageBuddy(page)) {
>> 			pfn++;
>> 			continue;
>> 		}
>>
>> 		/* Make sure we are not inadvertently changing nodes */
>> 		VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
>> 		VM_BUG_ON_PAGE(page_zone(page) != zone, page);
>>
>> 		order = buddy_order(page);
>> 		nr_pages = 1 << order;
>>
>> 		move_to_free_list(page, zone, order, old_mt, new_mt);
>>
>> 		pfn += nr_pages;
>> 		pages_moved += nr_pages;
>> 	}
>>
>> 	set_pageblock_migratetype(start_page, new_mt);
>>
>> 	return pages_moved;
>> }
>>
>> Does this look reasonable to you?
>
> Looks good to me. Thanks.
>
>>
>> Note that the page isolation specific stuff comes first. If this code
>> holds up, we should be able to move it to page-isolation.c and keep it
>> out of the regular allocator path.
>
> You mean move the tail and head part to set_migratetype_isolate()?
> And change move_freepages_block() to separate prep_move_freepages_block(),
> the tail and head code, and move_freepages()? It should work and looks
> like a similar code pattern as steal_suitable_fallback().

The attached patch has all the suggested changes, let me know how it
looks to you. Thanks.

--
Best Regards,
Yan, Zi

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