Messages in this thread |  | | Subject | Re: [RFC PATCH V2 2/10] mm: expose is_mem_section_removable() symbol | From | David Hildenbrand <> | Date | Fri, 10 Jan 2020 14:41:36 +0100 |
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On 07.01.20 14:36, Michal Hocko wrote: > On Tue 07-01-20 21:09:42, lantianyu1986@gmail.com wrote: >> From: Tianyu Lan <Tianyu.Lan@microsoft.com> >> >> Hyper-V balloon driver will use is_mem_section_removable() to >> check whether memory block is removable or not when receive >> memory hot remove msg. Expose it. > > I do not think this is a good idea. The check is inherently racy. Why > cannot the balloon driver simply hotremove the region when asked? >
It's not only racy, it also gives no guarantees. False postives and false negatives are possible.
If you want to avoid having to loop forever trying to offline when calling offline_and_remove_memory(), you could try to alloc_contig_range() the memory first and then play the PG_offline+notifier game like virtio-mem.
I don't remember clearly, but I think pinned pages can make offlining loop for a long time. And I remember there were other scenarios as well (including out of memory conditions and similar).
I sent an RFC [1] for powerpc/memtrace that does the same (just error handling is more complicated as it wants to offline and remove multiple consecutive memory blocks) - if you want to try to go down that path.
[1] https://lkml.kernel.org/r/20191217123851.8854-1-david@redhat.com
-- Thanks,
David / dhildenb
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