Subject: Re: [PATCH v4 4/7] mm: make alloc_contig_range work at pageblock granularity

From: Oscar Salvador <[email-masked]>
Date: Fri, 4 Feb 2022 14:56:52 +0100

On Wed, Jan 19, 2022 at 02:06:20PM -0500, Zi Yan wrote:
> From: Zi Yan <[email-masked]>
> 
> alloc_contig_range() worked at MAX_ORDER-1 granularity to avoid merging
> pageblocks with different migratetypes. It might unnecessarily convert
> extra pageblocks at the beginning and at the end of the range. Change
> alloc_contig_range() to work at pageblock granularity.
> 
> It is done by restoring pageblock types and split >pageblock_order free
> pages after isolating at MAX_ORDER-1 granularity and migrating pages
> away at pageblock granularity. The reason for this process is that
> during isolation, some pages, either free or in-use, might have >pageblock
> sizes and isolating part of them can cause free accounting issues.
> Restoring the migratetypes of the pageblocks not in the interesting
> range later is much easier.

Hi Zi Yan,

Due to time constraints I only glanced over, so some comments below
about stuff that caught my eye:

> +static inline void split_free_page_into_pageblocks(struct page *free_page,
> +				int order, struct zone *zone)
> +{
> +	unsigned long pfn;
> +
> +	spin_lock(&zone->lock);
> +	del_page_from_free_list(free_page, zone, order);
> +	for (pfn = page_to_pfn(free_page);
> +	     pfn < page_to_pfn(free_page) + (1UL << order);

It migt make sense to have a end_pfn variable so that does not have to
be constantly evaluated. Or maybe the compiler is clever enough to only
evualuate it once.

> +	     pfn += pageblock_nr_pages) {
> +		int mt = get_pfnblock_migratetype(pfn_to_page(pfn), pfn);
> +
> +		__free_one_page(pfn_to_page(pfn), pfn, zone, pageblock_order,
> +				mt, FPI_NONE);
> +	}
> +	spin_unlock(&zone->lock);

It is possible that free_page's order is already pageblock_order, so I
would add a one-liner upfront to catch that case and return, otherwise
we do the delete_from_freelist-and-free_it_again dance.

> +	/* Save the migratepages of the pageblocks before start and after end */
> +	num_pageblock_to_save = (alloc_start - isolate_start) / pageblock_nr_pages
> +				+ (isolate_end - alloc_end) / pageblock_nr_pages;
> +	saved_mt =
> +		kmalloc_array(num_pageblock_to_save,
> +			      sizeof(unsigned char), GFP_KERNEL);
> +	if (!saved_mt)
> +		return -ENOMEM;
> +
> +	num = save_migratetypes(saved_mt, isolate_start, alloc_start);
> +
> +	num = save_migratetypes(&saved_mt[num], alloc_end, isolate_end);

I really hope we can put all this magic within start_isolate_page_range,
and the counterparts in undo_isolate_page_range.

Also, I kinda dislike the &saved_mt thing. I thought about some other
approaches but nothing that wasn't too specific for this case, and I
guess we want that function to be as generic as possible.

> +	/*
> +	 * Split free page spanning [alloc_end, isolate_end) and put the
> +	 * pageblocks in the right migratetype list
> +	 */
> +	for (outer_end = alloc_end; outer_end < isolate_end;) {
> +		unsigned long begin_pfn = outer_end;
> +
> +		order = 0;
> +		while (!PageBuddy(pfn_to_page(outer_end))) {
> +			if (++order >= MAX_ORDER) {
> +				outer_end = begin_pfn;
> +				break;
> +			}
> +			outer_end &= ~0UL << order;
> +		}
> +
> +		if (outer_end != begin_pfn) {
> +			order = buddy_order(pfn_to_page(outer_end));
> +
> +			/*
> +			 * split the free page has start page and put the pageblocks
> +			 * in the right migratetype list
> +			 */
> +			VM_BUG_ON(outer_end + (1UL << order) <= begin_pfn);

How could this possibily happen?

> +			{
> +				struct page *free_page = pfn_to_page(outer_end);
> +
> +				split_free_page_into_pageblocks(free_page, order, cc.zone);
> +			}
> +			outer_end += 1UL << order;
> +		} else
> +			outer_end = begin_pfn + 1;
>  	}

I think there are cases could optimize for. If the page has already been
split in pageblock by the outer_start loop, we could skip this outer_end
logic altogether.

E.g: An order-10 page is split in two pageblocks. There's nothing else
to be done, right? We could skip this. 

-- 
Oscar Salvador
SUSE Labs