linux-kernel - Re: [PATCH 4/4] mm/hugeltb: handle the error case in hugetlb_fix_reserve

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Date:   Thu, 8 Apr 2021 16:25:31 -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 4/4] mm/hugeltb: handle the error case in
 hugetlb_fix_reserve_counts()

On 4/2/21 2:32 AM, Miaohe Lin wrote:
> A rare out of memory error would prevent removal of the reserve map region
> for a page. hugetlb_fix_reserve_counts() handles this rare case to avoid
> dangling with incorrect counts. Unfortunately, hugepage_subpool_get_pages
> and hugetlb_acct_memory could possibly fail too. We should correctly handle
> these cases.

Yes, this is a potential issue.

The 'good news' is that hugetlb_fix_reserve_counts() is unlikely to ever
be called.  To do so would imply we could not allocate a region entry
which is only 6 words in size.  We also keep a 'cache' of entries so we
may not even need to allocate.

But, as mentioned it is a potential issue.

> Fixes: b5cec28d36f5 ("hugetlbfs: truncate_hugepages() takes a range of pages")

This is likely going to make this get picked by by stable releases.
That is unfortunate as mentioned above this is mostly theoretical.

> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/hugetlb.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bdff8d23803f..ca5464ed04b7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -745,13 +745,20 @@ void hugetlb_fix_reserve_counts(struct inode *inode)
>  {
>  	struct hugepage_subpool *spool = subpool_inode(inode);
>  	long rsv_adjust;
> +	bool reserved = false;
>  
>  	rsv_adjust = hugepage_subpool_get_pages(spool, 1);
> -	if (rsv_adjust) {
> +	if (rsv_adjust > 0) {
>  		struct hstate *h = hstate_inode(inode);
>  
> -		hugetlb_acct_memory(h, 1);
> +		if (!hugetlb_acct_memory(h, 1))
> +			reserved = true;
> +	} else if (!rsv_adjust) {
> +		reserved = true;
>  	}
> +
> +	if (!reserved)
> +		pr_warn("hugetlb: fix reserve count failed\n");

We should expand this warning message a bit to indicate what this may
mean to the user.  Add something like"
	"Huge Page Reserved count may go negative".
-- 
Mike Kravetz