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Date:   Thu, 9 Jul 2020 08:43:40 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Joonsoo Kim <js1304@gmail.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        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>
Subject: Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA
 aware
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On Wed 08-07-20 09:41:06, 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:
> > > On 7/7/20 9:44 AM, js1304@gmail.com wrote:
> > > > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > > 
> > > > new_non_cma_page() in gup.c which try to allocate migration target page
> > > > requires to allocate the new page that is not on the CMA area.
> > > > new_non_cma_page() implements it by removing __GFP_MOVABLE flag.  This way
> > > > works well for THP page or normal page but not for hugetlb page.
> > > > 
> > > > hugetlb page allocation process consists of two steps.  First is dequeing
> > > > from the pool.  Second is, if there is no available page on the queue,
> > > > allocating from the page allocator.
> > > > 
> > > > new_non_cma_page() can control allocation from the page allocator by
> > > > specifying correct gfp flag.  However, dequeing cannot be controlled until
> > > > now, so, new_non_cma_page() skips dequeing completely.  It is a suboptimal
> > > > since new_non_cma_page() cannot utilize hugetlb pages on the queue so this
> > > > patch tries to fix this situation.
> > > > 
> > > > This patch makes the deque function on hugetlb CMA aware and skip CMA
> > > > pages if newly added skip_cma argument is passed as true.
> > > 
> > > Hmm, can't you instead change dequeue_huge_page_node_exact() to test the PF_
> > > flag and avoid adding bool skip_cma everywhere?
> > 
> > Okay! Please check following patch.
> > > 
> > > I think that's what Michal suggested [1] except he said "the code already does
> > > by memalloc_nocma_{save,restore} API". It needs extending a bit though, AFAICS.
> > > __gup_longterm_locked() indeed does the save/restore, but restore comes before
> > > check_and_migrate_cma_pages() and thus new_non_cma_page() is called, so an
> > > adjustment is needed there, but that's all?
> > > 
> > > Hm the adjustment should be also done because save/restore is done around
> > > __get_user_pages_locked(), but check_and_migrate_cma_pages() also calls
> > > __get_user_pages_locked(), and that call not being between nocma save and
> > > restore is thus also a correctness issue?
> > 
> > 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")
> 
> I am not sure this is worth backporting to stable yet.

Should I post it as a separate patch do you plan to include this into your next version?

> 
> 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:
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs