On 17.02.21 14:50, Michal Hocko wrote:

On Wed 17-02-21 14:36:47, David Hildenbrand wrote:

quoted

On 17.02.21 14:30, Michal Hocko wrote:

quoted

On Wed 17-02-21 11:08:15, Oscar Salvador wrote:

quoted

Free hugetlb pages are tricky to handle so as to no userspace application
notices disruption, we need to replace the current free hugepage with
a new one.

In order to do that, a new function called alloc_and_dissolve_huge_page
is introduced.
This function will first try to get a new fresh hugetlb page, and if it
succeeds, it will dissolve the old one.

With regard to the allocation, since we do not know whether the old page
was allocated on a specific node on request, the node the old page belongs
to will be tried first, and then we will fallback to all nodes containing
memory (N_MEMORY).

I do not think fallback to a different zone is ok. If yes then this
really requires a very good reasoning. alloc_contig_range is an
optimistic allocation interface at best and it shouldn't break carefully
node aware preallocation done by administrator.

What does memory offlining do when migrating in-use hugetlbfs pages? Does it
always keep the node?

No it will break the node pool. The reasoning behind that is that
offlining is an explicit request from the userspace and it is expected

userspace? in 99,9996% it's the hardware that triggers the unplug of a DIMM.

quoted

I think keeping the node is the easiest/simplest approach for now.

quoted

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Note that gigantic hugetlb pages are fenced off since there is a cyclic
dependency between them and alloc_contig_range.

Why do we need/want to do all this in the first place?

cma and virtio-mem (especially on ZONE_MOVABLE) really want to handle
hugetlbfs pages.

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.

-- 
Thanks,

David / dhildenb