Re: [RFC PATCH 1/8] hugetlb: add per-hstate mutex to synchronize user adjustments

[Michal Hocko]

On Fri 19-03-21 15:42:02, Mike Kravetz wrote:
> The number of hugetlb pages can be adjusted by writing to the
> sysps/proc files nr_hugepages, nr_hugepages_mempolicy or
> nr_overcommit_hugepages.  There is nothing to prevent two
> concurrent modifications via these files.  The underlying routine
> set_max_huge_pages() makes assumptions that only one occurrence is
> running at a time.  Specifically, alloc_pool_huge_page uses a
> hstate specific variable without any synchronization.

>From the above it is not really clear whether the unsynchronized nature
of set_max_huge_pages is really a problem or a mere annoynce. I suspect
the later because counters are properly synchronized with the
hugetlb_lock. It would be great to clarify that.
 
> Add a mutex to the hstate and use it to only allow one hugetlb
> page adjustment at a time.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  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 cccd1aab69dd..f42d44050548 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -555,6 +555,7 @@ 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 5b1ab1f427c5..d5be25f910e8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2601,6 +2601,8 @@ 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);
>  
>  	/*
> @@ -2633,6 +2635,7 @@ 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;
>  		}
> @@ -2707,6 +2710,7 @@ 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);
>  
> @@ -3194,6 +3198,7 @@ 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