linux-kernel - Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable hstates

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Date:   Wed, 27 Jan 2021 11:35:23 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable
 hstates

On Fri 22-01-21 11:52:29, Mike Kravetz wrote:
> The HP_Migratable flag indicates a page is a candidate for migration.
> Only set the flag if the page's hstate supports migration.  This allows
> the migration paths to detect non-migratable pages earlier.  If migration
> is not supported for the hstate, HP_Migratable will not be set, the page
> will not be isolated and no attempt will be made to migrate.  We should
> never get to unmap_and_move_huge_page for a page where migration is not
> supported, so throw a warning if we do.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  fs/hugetlbfs/inode.c    | 2 +-
>  include/linux/hugetlb.h | 9 +++++++++
>  mm/hugetlb.c            | 8 ++++----
>  mm/migrate.c            | 9 ++++-----
>  4 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index e1d7ed2a53a9..93f7b8d3c5fd 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -735,7 +735,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  
>  		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>  
> -		SetHPageMigratable(page);
> +		SetHPageMigratableIfSupported(page);
>  		/*
>  		 * unlock_page because locked by add_to_page_cache()
>  		 * put_page() due to reference from alloc_huge_page()
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 58be44a915d1..cd1960541f2a 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -740,6 +740,15 @@ static inline bool hugepage_migration_supported(struct hstate *h)
>  	return arch_hugetlb_migration_supported(h);
>  }
>  
> +/*
> + * Only set HPageMigratable if migration supported for page
> + */
> +static inline void SetHPageMigratableIfSupported(struct page *page)

This is really mouthful...

> +{
> +	if (hugepage_migration_supported(page_hstate(page)))
> +		SetHPageMigratable(page);

and it is really a trivial wrapper. I do understand why you want to
prevent from the code duplication and potentially a missing check but
this all is just an internal hugetlb code. Even if the flag is set on
non-migrateable hugetlb page then this will not be fatal. The migration
can fail even on those pages for which migration is supported right?

So I am not really sure this is an improvement in the end. But up to you
I do not really have a strong opinion here.

-- 
Michal Hocko
SUSE Labs