linux-kernel - Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe DEBUG

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Message-ID: <YLlOPoP/rIRMm2U5@t490s>
Date:   Thu, 3 Jun 2021 17:48:46 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Wang Yugui <wangyugui@...-tech.com>,
        Matthew Wilcox <willy@...radead.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>, Zi Yan <ziy@...dia.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>, Jue Wang <juew@...gle.com>,
        Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] mm/thp: try_to_unmap() use TTU_SYNC for safe
 DEBUG_VM splitting

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