On 17.02.21 14:59, Michal Hocko wrote:

On Wed 17-02-21 14:53:37, David Hildenbrand wrote:

quoted

On 17.02.21 14:50, Michal Hocko wrote:

[...]

quoted

quoted

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.

-- 
Thanks,

David / dhildenb