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Date:   Wed, 17 Feb 2021 15:14:46 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages
Message-ID: <YC0k1u3QtyU00kxC@dhcp22.suse.cz>
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On Wed 17-02-21 15:08:04, David Hildenbrand wrote:
> On 17.02.21 14:59, Michal Hocko wrote:
> > On Wed 17-02-21 14:53:37, David Hildenbrand wrote:
> > > On 17.02.21 14:50, Michal Hocko wrote:
> > [...]
> > > > Do we have any real life examples? Or does this fall more into, let's
> > > > optimize an existing implementation category.
> > > > 
> > > 
> > > It's a big TODO item I have on my list and I am happy that Oscar is looking
> > > into it. So yes, I noticed it while working on virtio-mem. It's real.
> > 
> > Do not take me wrong, I am not opposing to the functionality. I am
> > asking for the specific usecase.
> 
> Makes sense, and a proper motivation should be included in the patches/cover
> letter. So here comes a quick-n-dirty example:
> 
> 
> Start a VM with 4G. Hotplug 1G via virtio-mem and online it to ZONE_MOVABLE.
> Allocate 512 huge pages.
> 
> [root@localhost ~]# cat /proc/meminfo
> MemTotal:        5061512 kB
> MemFree:         3319396 kB
> MemAvailable:    3457144 kB
> ...
> HugePages_Total:     512
> HugePages_Free:      512
> HugePages_Rsvd:        0
> HugePages_Surp:        0
> Hugepagesize:       2048 kB
> 
> 
> The huge pages get partially allocate from ZONE_MOVABLE. Try unplugging 1G
> via virtio-mem (remember, all ZONE_MOVABLE). Inside the guest:
> 
> [  180.058992] alloc_contig_range: [1b8000, 1c0000) PFNs busy
> [  180.060531] alloc_contig_range: [1b8000, 1c0000) PFNs busy
> [  180.061972] alloc_contig_range: [1b8000, 1c0000) PFNs busy
> [  180.063413] alloc_contig_range: [1b8000, 1c0000) PFNs busy
> [  180.064838] alloc_contig_range: [1b8000, 1c0000) PFNs busy
> [  180.065848] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
> [  180.066794] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
> [  180.067738] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
> [  180.068669] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
> [  180.069598] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
> 
> 
> I succeed in unplugging 540MB - 484 MB remain blocked by huge pages ("which
> did not end up there by pure luck"). These pages are movable (and even
> free!) and can easily be reallocated.

OK, this sounds reasonable.
-- 
Michal Hocko
SUSE Labs