linux-kernel - Re: [PATCH 3/4] mm/rmap: fix new bug: premature return from page_mlock

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Date:   Thu, 8 Jul 2021 09:14:03 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Christoph Hellwig <hch@....de>, Yang Shi <shy828301@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 3/4] mm/rmap: fix new bug: premature return from page_mlock_one()

Thanks Hugh, evidently I missed this when re-adding the VM_LOCKED check
back.

Reviewed-by: Alistair Popple <apopple@...dia.com>

On Thursday, 8 July 2021 6:11:24 AM AEST Hugh Dickins wrote:
> In the unlikely race case that page_mlock_one() finds VM_LOCKED has been
> cleared by the time it got page table lock, page_vma_mapped_walk_done()
> must be called before returning, either explicitly, or by a final call
> to page_vma_mapped_walk() - otherwise the page table remains locked.
> 
> Fixes: cd62734ca60d ("mm/rmap: split try_to_munlock from try_to_unmap")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>  mm/rmap.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0e83c3be8568..1235368f0628 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1990,14 +1990,13 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
>  			 * this function is never called when PageDoubleMap().
>  			 */
>  			mlock_vma_page(page);
> +			/*
> +			 * No need to scan further once the page is marked
> +			 * as mlocked.
> +			 */
>  			page_vma_mapped_walk_done(&pvmw);
> +			return false;
>  		}
> -
> -		/*
> -		 * no need to continue scanning other vma's if the page has
> -		 * been locked.
> -		 */
> -		return false;
>  	}
>  
>  	return true;
>