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Subject: Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA
 aware
To:     Michal Hocko <mhocko@kernel.org>, Joonsoo Kim <js1304@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Christoph Hellwig <hch@infradead.org>,
        Roman Gushchin <guro@fb.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
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 <20200708071602.GB16543@js1304-desktop>
 <20200708074103.GD7271@dhcp22.suse.cz>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <8549326e-7485-dd6d-1fa1-a899228b9b2f@suse.cz>
Date:   Wed, 8 Jul 2020 11:26:54 +0200
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On 7/8/20 9:41 AM, Michal Hocko wrote:
> On Wed 08-07-20 16:16:02, Joonsoo Kim wrote:
>> On Tue, Jul 07, 2020 at 01:22:31PM +0200, Vlastimil Babka wrote:
>> 
>> Simply, I call memalloc_nocma_{save,restore} in new_non_cma_page(). It
>> would not cause any problem.
> 
> I believe a proper fix is the following. The scope is really defined for
> FOLL_LONGTERM pins and pushing it inside check_and_migrate_cma_pages
> will solve the problem as well but it imho makes more sense to do it in
> the caller the same way we do for any others. 
> 
> Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region")

Agreed.

> 
> I am not sure this is worth backporting to stable yet.

CC Aneesh.

Context: since check_and_migrate_cma_pages() calls __get_user_pages_locked(), it
should also be called under memalloc_nocma_save().

> diff --git a/mm/gup.c b/mm/gup.c
> index de9e36262ccb..75980dd5a2fc 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1794,7 +1794,6 @@ static long __gup_longterm_locked(struct task_struct *tsk,
>  				     vmas_tmp, NULL, gup_flags);
>  
>  	if (gup_flags & FOLL_LONGTERM) {
> -		memalloc_nocma_restore(flags);
>  		if (rc < 0)
>  			goto out;
>  
> @@ -1802,11 +1801,13 @@ static long __gup_longterm_locked(struct task_struct *tsk,
>  			for (i = 0; i < rc; i++)
>  				put_page(pages[i]);
>  			rc = -EOPNOTSUPP;
> +			memalloc_nocma_restore(flags);
>  			goto out;
>  		}
>  
>  		rc = check_and_migrate_cma_pages(tsk, mm, start, rc, pages,
>  						 vmas_tmp, gup_flags);
> +		memalloc_nocma_restore(flags);
>  	}
>  
>  out:
>