[RFC,V2,2/10] mm: expose is_mem_section_removable() symbol

Commit Message

Tianyu Lan Jan. 7, 2020, 1:09 p.m. UTC

From: Tianyu Lan <[e-mail]>

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.

Signed-off-by: Tianyu Lan <[e-mail]>
---
 mm/memory_hotplug.c | 1 +
 1 file changed, 1 insertion(+)

Comments

Michal Hocko Jan. 7, 2020, 1:36 p.m. UTC | #1

On Tue 07-01-20 21:09:42, [e-mail] wrote:
> From: Tianyu Lan <[e-mail]>
> 
> 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?

David Hildenbrand Jan. 10, 2020, 1:41 p.m. UTC | #2

On 07.01.20 14:36, Michal Hocko wrote:
> On Tue 07-01-20 21:09:42, [e-mail] wrote:
>> From: Tianyu Lan <[e-mail]>
>>
>> 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/[e-mail]

Tianyu Lan Jan. 13, 2020, 2:49 p.m. UTC | #3

> From: David Hildenbrand <[e-mail]>
> Sent: Friday, January 10, 2020 9:42 PM
> To: Michal Hocko <[e-mail]>; [e-mail]
> Cc: KY Srinivasan <[e-mail]>; Haiyang Zhang
> <[e-mail]>; Stephen Hemminger <[e-mail]>;
> [e-mail]; [e-mail]; Michael Kelley
> <[e-mail]>; Tianyu Lan <[e-mail]>; linux-
> [e-mail]; [e-mail]; [e-mail];
> vkuznets <[e-mail]>; [e-mail]; [e-mail];
> [e-mail]; Pasha Tatashin <[e-mail]>;
> [e-mail]
> 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, [e-mail] wrote:
> >> From: Tianyu Lan <[e-mail]>
> >>
> >> 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.

David Hildenbrand Jan. 13, 2020, 3:01 p.m. UTC | #4

On 13.01.20 15:49, Tianyu Lan wrote:
>> From: David Hildenbrand <[e-mail]>
>> Sent: Friday, January 10, 2020 9:42 PM
>> To: Michal Hocko <[e-mail]>; [e-mail]
>> Cc: KY Srinivasan <[e-mail]>; Haiyang Zhang
>> <[e-mail]>; Stephen Hemminger <[e-mail]>;
>> [e-mail]; [e-mail]; Michael Kelley
>> <[e-mail]>; Tianyu Lan <[e-mail]>; linux-
>> [e-mail]; [e-mail]; [e-mail];
>> vkuznets <[e-mail]>; [e-mail]; [e-mail];
>> [e-mail]; Pasha Tatashin <[e-mail]>;
>> [e-mail]
>> 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, [e-mail] wrote:
>>>> From: Tianyu Lan <[e-mail]>
>>>>
>>>> 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.

So it's all about the logging output. Duplicating these checks feels
very wrong. And you will still get plenty of page dumps (read below), so
that won't help.

We have pr_debug() for these "failure ..." message. that should
therefore not be an issue on production systems, right?

However, you will see dump_page()s quite often, which logs via pr_warn().

Of course, we could add a mechanism to temporarily disable logging
output for these call paths, but it might actually be helpful for
debugging. We might just want to convert everything that is not actually
a warning to pr_debug() - especially in dump_page().

Michal Hocko Jan. 14, 2020, 9:50 a.m. UTC | #5

On Mon 13-01-20 14:49:38, Tianyu Lan wrote:
> 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.

Why is a log message a problem in the first place. The operation has
failed afterall. Does the driver try to offline an arbitrary memory?
Could you describe your usecase in more details please?

Tianyu Lan Jan. 17, 2020, 4:35 p.m. UTC | #6

> From: Michal Hocko <[e-mail]>
> Sent: Tuesday, January 14, 2020 5:51 PM
> To: Tianyu Lan <[e-mail]>
> Cc: David Hildenbrand <[e-mail]>; [e-mail]; KY
> Srinivasan <[e-mail]>; Haiyang Zhang <[e-mail]>;
> Stephen Hemminger <[e-mail]>; [e-mail];
> [e-mail]; Michael Kelley <[e-mail]>; linux-
> [e-mail]; [e-mail]; [e-mail];
> vkuznets <[e-mail]>; [e-mail]; [e-mail];
> [e-mail]; Pasha Tatashin <[e-mail]>;
> [e-mail]
> Subject: Re: [EXTERNAL] Re: [RFC PATCH V2 2/10] mm: expose
> is_mem_section_removable() symbol
> 
> On Mon 13-01-20 14:49:38, Tianyu Lan wrote:
> > 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.
> 
> Why is a log message a problem in the first place. The operation has failed
> afterall. Does the driver try to offline an arbitrary memory?

Yes.

> Could you describe your usecase in more details please?

Hyper-V sends hot-remove request message which just contains requested
page number but not provide detail range. So Hyper-V driver needs to search
suitable memory block in system memory to return back to host if there is no
memory hot-add before. So I used the is_mem_section_removable() do such check.

Michal Hocko Jan. 20, 2020, 2:14 p.m. UTC | #7

On Fri 17-01-20 16:35:03, Tianyu Lan wrote:
[...]
> > Could you describe your usecase in more details please?
> 
> Hyper-V sends hot-remove request message which just contains requested
> page number but not provide detail range. So Hyper-V driver needs to search
> suitable memory block in system memory to return back to host if there is no
> memory hot-add before. So I used the is_mem_section_removable() do such check.

As David described, you would be much better of by using
alloc_contig_range to find a memory that would be suitable for
hotremoving without any races.

Patch

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index d04369e6d3cc..a4ebfc5c48b3 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1179,6 +1179,7 @@  bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
 	/* All pageblocks in the memory block are likely to be hot-removable */
 	return true;
 }
+EXPORT_SYMBOL_GPL(is_mem_section_removable);
 
 /*
  * Confirm all pages in a range [start, end) belong to the same zone.