RE: [EXTERNAL] Re: [RFC PATCH V2 2/10] mm: expose is_mem_section_removable() symbol

[Tianyu Lan]

> 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.