Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 26 Jun 2020 09:23:24 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Joonsoo Kim <js1304@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@lge.com,
        Vlastimil Babka <vbabka@suse.cz>,
        Christoph Hellwig <hch@infradead.org>,
        Roman Gushchin <guro@fb.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v3 4/8] mm/hugetlb: make hugetlb migration callback CMA
 aware
Message-ID: <20200626072324.GT1320@dhcp22.suse.cz>
References: <1592892828-1934-1-git-send-email-iamjoonsoo.kim@lge.com>
 <1592892828-1934-5-git-send-email-iamjoonsoo.kim@lge.com>
 <20200625115422.GE1320@dhcp22.suse.cz>
 <CAAmzW4MHuRhNqVXMntLAc_x4kJgkgQ-pD5GfFxRxJRchrEFr9g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAAmzW4MHuRhNqVXMntLAc_x4kJgkgQ-pD5GfFxRxJRchrEFr9g@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri 26-06-20 13:49:15, Joonsoo Kim wrote:
> 2020년 6월 25일 (목) 오후 8:54, Michal Hocko <mhocko@kernel.org>님이 작성:
> >
> > On Tue 23-06-20 15:13:44, Joonsoo Kim 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.
> >
> > Could you explain why? I mean why cannot you simply remove __GFP_MOVABLE
> > flag when calling alloc_huge_page_nodemask and check for it in dequeue
> > path?
> 
> If we remove __GFP_MOVABLE when calling alloc_huge_page_nodemask, we cannot
> use the page in ZONE_MOVABLE on dequeing.
> 
> __GFP_MOVABLE is not only used for CMA selector but also used for zone
> selector.  If we clear it, we cannot use the page in the ZONE_MOVABLE
> even if it's not CMA pages.  For THP page or normal page allocation,
> there is no way to avoid this weakness without introducing another
> flag or argument. For me, introducing another flag or argument for
> these functions looks over-engineering so I don't change them and
> leave them as they are (removing __GFP_MOVABLE).
> 
> But, for alloc_huge_page_nodemask(), introducing a new argument
> doesn't seem to be a problem since it is not a general function but
> just a migration target allocation function.

I really do not see why hugetlb and only the dequeing part should be
special. This just leads to a confusion. From the code point of view it
makes perfect sense to opt out CMA regions for !__GFP_MOVABLE when
dequeing. So I would rather see a consistent behavior than a special
case deep in the hugetlb allocator layer.
-- 
Michal Hocko
SUSE Labs