linux-kernel - Re: [PATCH 2/6] hugetlbfs: fix cannot migrate the fallocated HugeTLB page

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Message-ID: <09be227a-4e16-1960-c8e9-609c35a80ef5@oracle.com>
Date:   Mon, 4 Jan 2021 14:38:03 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org
Cc:     hillf.zj@...baba-inc.com, n-horiguchi@...jp.nec.com,
        ak@...ux.intel.com, yongjun_wei@...ndmicro.com.cn, mhocko@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] hugetlbfs: fix cannot migrate the fallocated HugeTLB
 page

On 1/3/21 10:58 PM, Muchun Song wrote:
> Because we only can isolate a active page via isolate_huge_page()
> and hugetlbfs_fallocate() forget to mark it as active, we cannot
> isolate and migrate those pages.
> 
> Fixes: 70c3547e36f5 (hugetlbfs: add hugetlbfs_fallocate())
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  fs/hugetlbfs/inode.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

Good catch.  This is indeed an issue.

> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index b5c109703daa..2aceb085d202 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -737,10 +737,11 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  
>  		/*
>  		 * unlock_page because locked by add_to_page_cache()
> -		 * page_put due to reference from alloc_huge_page()
> +		 * put_page() (which is in the putback_active_hugepage())
> +		 * due to reference from alloc_huge_page()

Thanks for fixing the comment.

>  		 */
>  		unlock_page(page);
> -		put_page(page);
> +		putback_active_hugepage(page);

I'm curious why you used putback_active_hugepage() here instead of simply
calling set_page_huge_active() before the put_page()?

When the page was allocated, it was placed on the active list (alloc_huge_page).
Therefore, the hugetlb_lock locking and list movement should not be necessary.

-- 
Mike Kravetz

>  	}
>  
>  	if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
>