On 25.03.21 13:35, Michal Hocko wrote:
On Thu 25-03-21 12:08:43, David Hildenbrand wrote:
On 25.03.21 11:55, Oscar Salvador wrote:
On Thu, Mar 25, 2021 at 10:17:33AM +0100, Michal Hocko wrote:
Why do you think it is wrong to initialize/account pages when they are
used? Keep in mind that offline pages are not used until they are
onlined. But vmemmap pages are used since the vmemmap is established
which happens in the hotadd stage.
Yes, that is true.
vmemmap pages are used right when we populate the vmemmap space.
Note: I once herd of a corner-case use case where people offline memory
blocks to then use the "free" memory via /dev/mem for other purposes ("large
physical memory"). Not that I encourage such use cases, but they would be
fundamentally broken if the vmemmap ends up on offline memory and is
supposed to keep its state ...
I am not aware of such a use case, it surely sounds, ehm creative, but
nothing really new. But such a usecase sounds quite incompatible with
this feature whether we account vmemmap at hotadd or hotremove because
they would need to understand that part of the memory they have hotadded
is not useable.
I think they can use it just fine via /dev/mem, which explicitly avoids
any kind of "struct page" references IIRC. They would be overwriting the
vmemmap, but that part scan happily read/write until onlining, where the
vmemmap would get reinitialized and set online - from which point on
pfn_to_online_page() would succeed also on the vmemmap itself.
[...]
- When moving the initialization/accounting to hot-add/hot-remove,
the section containing the vmemmap pages will remain offline.
It might get onlined once the pages get online in online_pages(),
or not if vmemmap pages span a whole section.
I remember (but maybe David rmemeber better) that that was a problem
wrt. pfn_to_online_page() and hybernation/kdump.
So, if that is really a problem, we would have to care of ot setting
the section to the right state.
Good memory. Indeed, hibernation/kdump won't save the state of the vmemmap,
because the memory is marked as offline and, thus, logically without any
valuable content.
Could you point me to the respective hibernation code please? I always
get lost in that area. Anyway, we do have the same problem even if the
whole accounting is handled during {on,off}lining, no?
kernel/power/snapshot.c:saveable_page().
I am not really worried about kdump though. As the memory is offline
then losing itse vmemmap is a mere annoyance.
Yep, kdump was relevant in our previous discussions when we were talking
about memory blocks having their altmap located on other memory blocks.
- AFAICS, doing all the above brings us to former times were some
initialization/accounting was done in a previous stage, and I remember
it was pushed hard to move those in online/offline_pages().
Are we ok with that?
As I said, we might have to set the right zone in hot-add stage, as
otherwise problems might come up.
Being that case, would not that also be conflating different concepts
at a wrong phases?
I expressed my opinion already, no need to repeat. Sub-section online maps
would make it cleaner, but I am still not convinced we want/need that.
Nah, subsections are more tricky than necessary and if we can live
without them and have it just as pmem weirdness is more than enough ;)
Yes, absolutely :)
--
Thanks,
David / dhildenb
