On 01.03.21 13:57, Oscar Salvador wrote:

On Mon, Mar 01, 2021 at 01:43:00PM +0100, David Hildenbrand wrote:

quoted

Same experiment with ZONE_MOVABLE:

a) Free huge pages: all memory can get unplugged again.

b) Allocated/populated but idle huge pages: all memory can get unplugged
again.

c) Allocated/populated but all 512 huge pages are read/written in a loop:
all memory can get unplugged again, but I get a single

[  121.192345] alloc_contig_range: [180000, 188000) PFNs busy

Most probably because it happened to try migrating a huge page while it was
busy. As virtio-mem retries on ZONE_MOVABLE a couple of times, it can deal
with this temporary failure.



Last but not least, I did something extreme:

]# cat /proc/meminfo
MemTotal:        5061568 kB
MemFree:          186560 kB
MemAvailable:     354524 kB
...
HugePages_Total:    2048
HugePages_Free:     2048
HugePages_Rsvd:        0
HugePages_Surp:        0


Triggering unplug would require to dissolve+alloc - which now fails when
trying to allocate an additional ~512 huge pages (1G).


As expected, I can properly see memory unplug not fully succeeding. + I get
a fairly continuous stream of

[  226.611584] alloc_contig_range: [19f400, 19f800) PFNs busy
...

But more importantly, the hugepage count remains stable, as configured by
the admin (me):

HugePages_Total:    2048
HugePages_Free:     2048
HugePages_Rsvd:        0
HugePages_Surp:        0

Thanks for giving it a spin David, that is highly appreciated ;-)!

I will add above information in next's version changelog if you do not mind,
so the before-and-after can be seen clearly.

I shall send v4 in the course of the next few days.

I'll have some review feedback on error handling that might be improved, 
I'll share that shortly.

-- 
Thanks,

David / dhildenb