Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG_VM splitting

[Peter Xu]

On Tue, Jun 01, 2021 at 02:07:53PM -0700, Hugh Dickins wrote:
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 2cf01d933f13..b45d22738b45 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -212,6 +212,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			pvmw->ptl = NULL;
>  		}
>  	} else if (!pmd_present(pmde)) {
> +		/*
> +		 * If PVMW_SYNC, take and drop THP pmd lock so that we
> +		 * cannot return prematurely, while zap_huge_pmd() has
> +		 * cleared *pmd but not decremented compound_mapcount().
> +		 */
> +		if ((pvmw->flags & PVMW_SYNC) &&
> +		    PageTransCompound(pvmw->page))
> +			spin_unlock(pmd_lock(mm, pvmw->pmd));
>  		return false;
>  	}
>  	if (!map_pte(pvmw))

Sorry if I missed something important, but I'm totally confused on how this
unlock is pairing with another lock()..

And.. isn't PVMW_SYNC only meaningful for pte-level only (as I didn't see a
reference of it outside map_pte)?

-- 
Peter Xu