Re: [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages

[David Hildenbrand]

On 01.03.21 13:57, Oscar Salvador wrote:
> On Mon, Mar 01, 2021 at 01:43:00PM +0100, David Hildenbrand wrote:
> > 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