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

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Date:   Thu, 25 Mar 2021 13:29:40 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        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, Mar 24, 2021 at 05:28:30PM -0700, 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.

Is this something you have seen happening? If so, it is easier to
trigger? I doubt so as I have not seen any bug report, but just
wondering whether a Fixes tag is needed, or probably not worth, right?

> --- 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;

I am also with Michal here, renaming the mutex to something closer to
its function might be better to understand it without diving too much in
the code.

>  	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);

I find above comment a bit misleading.
AFAIK, hugetlb_lock also protects from concurrent adjustments for the
same hstate (hugepage_activelist, free_huge_pages, surplus_huge_pages,
etc...).
Would it be more apropiate saying that mutex_lock() only prevents from
simultaneously sysfs/proc operations?

Reviewed-by: Oscar Salvador <osalvador@...e.e>


-- 
Oscar Salvador
SUSE L3