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Subject: Re: [PATCH 5/5] mm/hugetl.c: warn out if expected count of huge pages
 adjustment is not achieved
To:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org
References: <20200720062623.13135-1-bhe@redhat.com>
 <20200720062623.13135-6-bhe@redhat.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <c5eb3692-2d05-d6dc-437d-21e51705560e@oracle.com>
Date:   Mon, 20 Jul 2020 17:38:59 -0700
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On 7/19/20 11:26 PM, Baoquan He wrote:
> A customer complained that no any message is printed out when failed to
> allocate explicitly specified number of persistent huge pages. That
> specifying can be done by writing into /proc/sys/vm/nr_hugepages to
> increase the persisten huge pages.
> 
> In the current code, it takes the best effort way to allocate the expected
> number of huge pages. If only succeeding to get part of them, no any
> information is printed out.
> 
> Here try to send out warning message if the expected number of huge pages
> adjustment is not achieved, including increasing and decreasing the count
> of persistent huge pages.

Perhaps change the wording a bit,

A customer complained that no message is logged when the number of
persistent huge pages is not changed to the exact value written to
the sysfs or proc nr_hugepages file.

In the current code, a best effort is made to satisfy requests made
via the nr_hugepages file.  However, requests may be only partially
satisfied.

Log a message if the code was unsuccessful in fully satisfying a
request.  This includes both increasing and decreasing the number
of persistent huge pages.

> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  mm/hugetlb.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)

I am not opposed to this patch.  However, I believe the best way for a user
to determine if their request was successful is to compare the value of
nr_hugepages to the value which was written.

> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 467894d8332a..1dfb5d9e4e06 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2661,7 +2661,7 @@ static int adjust_pool_surplus(struct hstate *h, nodemask_t *nodes_allowed,
>  static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  			      nodemask_t *nodes_allowed)
>  {
> -	unsigned long min_count, ret;
> +	unsigned long min_count, ret, old_max;
>  	NODEMASK_ALLOC(nodemask_t, node_alloc_noretry, GFP_KERNEL);
>  
>  	/*
> @@ -2723,6 +2723,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	 * pool might be one hugepage larger than it needs to be, but
>  	 * within all the constraints specified by the sysctls.
>  	 */
> +	old_max = persistent_huge_pages(h);
>  	while (h->surplus_huge_pages && count > persistent_huge_pages(h)) {
>  		if (!adjust_pool_surplus(h, nodes_allowed, -1))
>  			break;
> @@ -2779,6 +2780,16 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	}
>  out:
>  	h->max_huge_pages = persistent_huge_pages(h);
> +	if (count != h->max_huge_pages) {
> +		char buf[32];
> +
> +		string_get_size(huge_page_size(h), 1, STRING_UNITS_2, buf, 32);
> +		pr_warn("HugeTLB: %s %lu of page size %s failed. Only %s %lu hugepages.\n",
> +			count > old_max ? "increasing" : "decreasing",
> +			abs(count - old_max), buf,
> +			count > old_max ? "increased" : "decreased",
> +			abs(old_max - h->max_huge_pages));
> +	}
>  	spin_unlock(&hugetlb_lock);

I would prefer if we drop the lock before logging the message.  That would
involve grabbing the value of h->max_huge_pages before dropping the lock.
-- 
Mike Kravetz

>  
>  	NODEMASK_FREE(node_alloc_noretry);
>