linux-kernel - Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for hugetlb_acct

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Message-ID: <20afccd5-2bc4-9db9-695e-dd6175b0b42b@oracle.com>
Date:   Tue, 6 Apr 2021 19:49:58 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     n-horiguchi@...jp.nec.com, hillf.zj@...baba-inc.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for
 hugetlb_acct_memory()

On 4/2/21 2:32 AM, Miaohe Lin wrote:
> The resv_map could be NULL since this routine can be called in the evict
> inode path for all hugetlbfs inodes. So we could have chg = 0 and this
> would result in a negative value when chg - freed. This is unexpected for
> hugepage_subpool_put_pages() and hugetlb_acct_memory().

I am not sure if this is possible.

It is true that resv_map could be NULL.  However, I believe resv map
can only be NULL for inodes that are not regular or link inodes.  This
is the inode creation code in hugetlbfs_get_inode().

       /*
         * Reserve maps are only needed for inodes that can have associated
         * page allocations.
         */
        if (S_ISREG(mode) || S_ISLNK(mode)) {
                resv_map = resv_map_alloc();
                if (!resv_map)
                        return NULL;
        }

If resv_map is NULL, then no hugetlb pages can be allocated/associated
with the file.  As a result, remove_inode_hugepages will never find any
huge pages associated with the inode and the passed value 'freed' will
always be zero.

Does that sound correct?

-- 
Mike Kravetz

> 
> Fixes: b5cec28d36f5 ("hugetlbfs: truncate_hugepages() takes a range of pages")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/hugetlb.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b7864abded3d..bdff8d23803f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5413,6 +5413,7 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
>  	long chg = 0;
>  	struct hugepage_subpool *spool = subpool_inode(inode);
>  	long gbl_reserve;
> +	long delta;
>  
>  	/*
>  	 * Since this routine can be called in the evict inode path for all
> @@ -5437,7 +5438,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
>  	 * If the subpool has a minimum size, the number of global
>  	 * reservations to be released may be adjusted.
>  	 */
> -	gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed));
> +	delta = chg > 0 ? chg - freed : freed;
> +	gbl_reserve = hugepage_subpool_put_pages(spool, delta);
>  	hugetlb_acct_memory(h, -gbl_reserve);
>  
>  	return 0;
>