Re: [RFC PATCH v2 3/7] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

[Eric Ren]

Hi,

On 2021/12/10 07:04, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> alloc_migration_target() is used by alloc_contig_range() and non-LRU
> movable compound pages can be migrated. Current code does not allocate the
> right page size for such pages. Check THP precisely using
> is_transparent_huge() and add allocation support for non-LRU compound
> pages.
Could you elaborate on why the current code doesn't get the right size?  
how this patch fixes it.

The description sounds like it's an existing bug, if so, the patch 
subject should be changed to
"Fixes ..."?

> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>   mm/migrate.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d487a399253b..2ce3c771b1de 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1563,7 +1563,7 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
>   		return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
>   	}
>   
> -	if (PageTransHuge(page)) {
> +	if (is_transparent_hugepage(page)) {
>   		/*
>   		 * clear __GFP_RECLAIM to make the migration callback
>   		 * consistent with regular THP allocations.
> @@ -1572,13 +1572,17 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
if (PageTransHuge(page)) {  // just give more code context
...
>   		gfp_mask |= GFP_TRANSHUGE;
>   		order = HPAGE_PMD_ORDER;
order assigned here
>   	}
> +	if (PageCompound(page)) {
> +		gfp_mask |= __GFP_COMP;
> +		order = compound_order(page);
re-assinged again as THP is a compound page?

Thanks,
Eric
> +	}
>   	zidx = zone_idx(page_zone(page));
>   	if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
>   		gfp_mask |= __GFP_HIGHMEM;
>   
>   	new_page = __alloc_pages(gfp_mask, order, nid, mtc->nmask);
>   
> -	if (new_page && PageTransHuge(new_page))
> +	if (new_page && is_transparent_hugepage(page))
>   		prep_transhuge_page(new_page);
>   
>   	return new_page;