Re: [PATCH 1/2] mm: Make alloc_contig_range handle free hugetlb pages

[Oscar Salvador]

On Wed, Feb 17, 2021 at 03:08:04PM +0100, 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:

Definitely. I took it for granted that the problem was obvious.

> 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.

Thanks for providing such a detailed case David.
I will gather the bits and prepare a v2 once I gather more feedback.

-- 
Oscar Salvador
SUSE L3