Re: [PATCH] mm/contig_alloc: fix alloc_contig_range when __GFP_COMP and order < MAX_ORDER

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 12 Mar 2025 16:47:05 +0800 Jinjiang Tu <tujinjiang@huawei.com> wrote:

> When calling alloc_contig_range() with __GFP_COMP and the order of
> requested pfn range is pageblock_order, less than MAX_ORDER, I triggered
> WARNING as follows:
> 
>  PFN range: requested [2150105088, 2150105600), allocated [2150105088, 2150106112)
>  WARNING: CPU: 3 PID: 580 at mm/page_alloc.c:6877 alloc_contig_range+0x280/0x340
> 
> alloc_contig_range() marks pageblocks of the requested pfn range to be
> isolated, migrate these pages if they are in use and will be freed to
> MIGRATE_ISOLATED freelist.
> 
> Suppose two alloc_contig_range() calls at the same time and the requested
> pfn range are [0x80280000, 0x80280200) and [0x80280200, 0x80280400)
> respectively. Suppose the two memory range are in use, then
> alloc_contig_range() will migrate and free these pages to MIGRATE_ISOLATED
> freelist. __free_one_page() will merge MIGRATE_ISOLATE buddy to larger
> buddy, resulting in a MAX_ORDER buddy. Finally, find_large_buddy() in
> alloc_contig_range() returns a MAX_ORDER buddy and results in WARNING.
> 
> To fix it, call free_contig_range() to free the excess pfn range.

This has been in mm-hotfixes for a month without issue.  Is there any
reviewer interest?

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6528,7 +6528,8 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>  		goto done;
>  	}
>  
> -	if (!(gfp_mask & __GFP_COMP)) {
> +	if (!(gfp_mask & __GFP_COMP) ||
> +		(is_power_of_2(end - start) && ilog2(end - start) < MAX_PAGE_ORDER)) {
>  		split_free_pages(cc.freepages, gfp_mask);
>  
>  		/* Free head and tail (if any) */
> @@ -6536,7 +6537,15 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>  			free_contig_range(outer_start, start - outer_start);
>  		if (end != outer_end)
>  			free_contig_range(end, outer_end - end);
> -	} else if (start == outer_start && end == outer_end && is_power_of_2(end - start)) {
> +
> +		outer_start = start;
> +		outer_end = end;
> +
> +		if (!(gfp_mask & __GFP_COMP))
> +			goto done;
> +	}
> +
> +	if (start == outer_start && end == outer_end && is_power_of_2(end - start)) {
>  		struct page *head = pfn_to_page(start);
>  		int order = ilog2(end - start);
>