Re: [PATCH 1/2] mm,memory_hotplug: Remove un-taken lock

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 22.05.24 16:27, Brendan Jackman wrote:
On Wed, May 22, 2024 at 04:09:41PM +0200, David Hildenbrand wrote:
On 21.05.24 14:57, Brendan Jackman wrote:
The old seqlock guaranteed that we would have obtained consistent values
here. start + spanned_pages defines a range. For example, growing a zone to
the beginning implies that both ranges must be changed.

I do wonder if it might be better to instead have zone->zone_start_pfn and
zone->zone_end_pfn. That way, both can be changed individually, not
requiring adjustment of both to grow/shrink a zone at the beginning.

Thanks this is a good point.

So basically the fact that spanned_pages is "once or eventually"
correct is certainly not enough because it only has meaning with
reference to zone_start_pfn. I didn't realise this because of my
spontaneous inspiration to believe that zone_start_pfn was fixed.

Right, it isn't.


By the way, some noob questions: am I OK with my assumption that it's
fine for reader code to operate on zone spans that are both stale and
"from the future"? thinking abstractly I guess that seeing a stale
value when racing with offline_pages is roughly the same as seeing a
value "from the future" when racing with online_pages?

Right. PFN walkers should be using pfn_to_online_page(), where races are possible but barely seen in practice.

zone handlers like mm/compaction.c can likely deal with races, although it might all be cleaner (and safer?) when using start+end. I recall it also recalls on pfn_to_online_page().

Regarding page_outside_zone_boundaries(), it should be fine if we can read start+end atomically, that way we would not accidentally report "page outside ..." when changing the start address. I think with your current patch that might happen (although likely extremely hard to trigger) when growing the zone at the start, reducing zone_start_pfn.


Also, is it ever possible for pages to get removed and then added back
and end up in a different zone than before?

Yes. Changing between MOVABLE and NORMAL is possible and can easily be triggered by offlining+re-onlining memory blocks.

--
Cheers,

David / dhildenb





[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux OMAP]     [Linux MIPS]     [eCos]     [Asterisk Internet PBX]     [Linux API]

  Powered by Linux