> From: David Hildenbrand <david@xxxxxxxxxx> > Sent: Friday, January 10, 2020 9:42 PM > To: Michal Hocko <mhocko@xxxxxxxxxx>; lantianyu1986@xxxxxxxxx > Cc: KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang > <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; > sashal@xxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; Michael Kelley > <mikelley@xxxxxxxxxxxxx>; Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>; linux- > hyperv@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; > vkuznets <vkuznets@xxxxxxxxxx>; eric.devolder@xxxxxxxxxx; vbabka@xxxxxxx; > osalvador@xxxxxxx; Pasha Tatashin <Pavel.Tatashin@xxxxxxxxxxxxx>; > rppt@xxxxxxxxxxxxx > Subject: [EXTERNAL] Re: [RFC PATCH V2 2/10] mm: expose > is_mem_section_removable() symbol > > On 07.01.20 14:36, Michal Hocko wrote: > > On Tue 07-01-20 21:09:42, lantianyu1986@xxxxxxxxx wrote: > >> From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> > >> > >> 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. > Hi David & Michal: Thanks for your review. Some memory blocks are not suitable for hot-plug. If not check memory block's removable, offline_pages() will report some failure error e.g, "failed due to memory holes" and "failure to isolate range". I think the check maybe added into offline_and_remove_memory()? This may help to not create/expose a new interface to do such check in module.
