Re: [PATCH V2 0/6] mm: page_alloc: freelist migratetype hygiene

[Zi Yan <ziy@nvidia.com>]

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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@nvidia.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@nvidia.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().


--
Best Regards,
Yan, Zi

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