linux-kernel - Re: [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page

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Message-ID: <20210106163513.GS13207@dhcp22.suse.cz>
Date:   Wed, 6 Jan 2021 17:35:13 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        n-horiguchi@...jp.nec.com, ak@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated
 HugeTLB page

On Wed 06-01-21 16:47:35, Muchun Song wrote:
> Because we only can isolate a active page via isolate_huge_page()
> and hugetlbfs_fallocate() forget to mark it as active, we cannot
> isolate and migrate those pages.

I've little bit hard time to understand this initially and had to dive
into the code to make sense of it. I would consider the following
wording easier to grasp. Feel free to reuse if you like.
"
If a new hugetlb page is allocated during fallocate it will not be
marked as active (set_page_huge_active) which will result in a later
isolate_huge_page failure when the page migration code would like to
move that page. Such a failure would be unexpected and wrong.
"

Now to the fix. I believe that this patch shows that the
set_page_huge_active is just too subtle. Is there any reason why we
cannot make all freshly allocated huge pages active by default?
 
> Only export set_page_huge_active, just leave clear_page_huge_active
> as static. Because there are no external users.
> 
> Fixes: 70c3547e36f5 (hugetlbfs: add hugetlbfs_fallocate())
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  fs/hugetlbfs/inode.c    | 3 ++-
>  include/linux/hugetlb.h | 2 ++
>  mm/hugetlb.c            | 2 +-
>  3 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index b5c109703daa..21c20fd5f9ee 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -735,9 +735,10 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  
>  		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>  
> +		set_page_huge_active(page);
>  		/*
>  		 * unlock_page because locked by add_to_page_cache()
> -		 * page_put due to reference from alloc_huge_page()
> +		 * put_page() due to reference from alloc_huge_page()
>  		 */
>  		unlock_page(page);
>  		put_page(page);
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index ebca2ef02212..b5807f23caf8 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -770,6 +770,8 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
>  }
>  #endif
>  
> +void set_page_huge_active(struct page *page);
> +
>  #else	/* CONFIG_HUGETLB_PAGE */
>  struct hstate {};
>  
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1f3bf1710b66..4741d60f8955 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1348,7 +1348,7 @@ bool page_huge_active(struct page *page)
>  }
>  
>  /* never called for tail page */
> -static void set_page_huge_active(struct page *page)
> +void set_page_huge_active(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
>  	SetPagePrivate(&page[1]);
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs