linux-kernel - Re: [PATCH 3/8] hugetlb: add per-hstate mutex to synchronize user adjustments

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Message-ID: <YFxqKH0OQoFMEiIb@dhcp22.suse.cz>
Date:   Thu, 25 Mar 2021 11:47:04 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Hillf Danton <hdanton@...a.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/8] hugetlb: add per-hstate mutex to synchronize user
 adjustments

On Wed 24-03-21 17:28:30, Mike Kravetz wrote:
> The helper routine hstate_next_node_to_alloc accesses and modifies the
> hstate variable next_nid_to_alloc.  The helper is used by the routines
> alloc_pool_huge_page and adjust_pool_surplus.  adjust_pool_surplus is
> called with hugetlb_lock held.  However, alloc_pool_huge_page can not
> be called with the hugetlb lock held as it will call the page allocator.
> Two instances of alloc_pool_huge_page could be run in parallel or
> alloc_pool_huge_page could run in parallel with adjust_pool_surplus
> which may result in the variable next_nid_to_alloc becoming invalid
> for the caller and pages being allocated on the wrong node.
> 
> Both alloc_pool_huge_page and adjust_pool_surplus are only called from
> the routine set_max_huge_pages after boot.  set_max_huge_pages is only
> called as the reusult of a user writing to the proc/sysfs nr_hugepages,
> or nr_hugepages_mempolicy file to adjust the number of hugetlb pages.
> 
> It makes little sense to allow multiple adjustment to the number of
> hugetlb pages in parallel.  Add a mutex to the hstate and use it to only
> allow one hugetlb page adjustment at a time.  This will synchronize
> modifications to the next_nid_to_alloc variable.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

I would just recommend s@...ex@...ize_lock@ so that the intention is
more clear from the name.
> ---
>  include/linux/hugetlb.h | 1 +
>  mm/hugetlb.c            | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index a7f7d5f328dc..8817ec987d68 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -566,6 +566,7 @@ HPAGEFLAG(Freed, freed)
>  #define HSTATE_NAME_LEN 32
>  /* Defines one hugetlb page size */
>  struct hstate {
> +	struct mutex mutex;
>  	int next_nid_to_alloc;
>  	int next_nid_to_free;
>  	unsigned int order;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f9ba63fc1747..404b0b1c5258 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2616,6 +2616,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	else
>  		return -ENOMEM;
>  
> +	/* mutex prevents concurrent adjustments for the same hstate */
> +	mutex_lock(&h->mutex);
>  	spin_lock(&hugetlb_lock);
>  
>  	/*
> @@ -2648,6 +2650,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  	if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
>  		if (count > persistent_huge_pages(h)) {
>  			spin_unlock(&hugetlb_lock);
> +			mutex_unlock(&h->mutex);
>  			NODEMASK_FREE(node_alloc_noretry);
>  			return -EINVAL;
>  		}
> @@ -2722,6 +2725,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
>  out:
>  	h->max_huge_pages = persistent_huge_pages(h);
>  	spin_unlock(&hugetlb_lock);
> +	mutex_unlock(&h->mutex);
>  
>  	NODEMASK_FREE(node_alloc_noretry);
>  
> @@ -3209,6 +3213,7 @@ void __init hugetlb_add_hstate(unsigned int order)
>  	BUG_ON(hugetlb_max_hstate >= HUGE_MAX_HSTATE);
>  	BUG_ON(order == 0);
>  	h = &hstates[hugetlb_max_hstate++];
> +	mutex_init(&h->mutex);
>  	h->order = order;
>  	h->mask = ~(huge_page_size(h) - 1);
>  	for (i = 0; i < MAX_NUMNODES; ++i)
> -- 
> 2.30.2
> 

-- 
Michal Hocko
SUSE Labs