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
