Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[David Hildenbrand]

On 25.11.20 20:01, Andrea Arcangeli wrote:
> On Wed, Nov 25, 2020 at 01:08:54PM +0100, Vlastimil Babka wrote:
>> Yeah I guess it would be simpler if zoneid/nid was correct for 
>> pfn_valid() pfns within a zone's range, even if they are reserved due 
>> not not being really usable memory.
>>
>> I don't think we want to introduce CONFIG_HOLES_IN_ZONE to x86. If the 
>> chosen solution is to make this to a real hole, the hole should be 
>> extended to MAX_ORDER_NR_PAGES aligned boundaries.
> 
> The way pfn_valid works it's not possible to render all non-RAM pfn as
> !pfn_valid, CONFIG_HOLES_IN_ZONE would not achieve it 100% either. So

Well, we could do it the arm64 way and provide a custom pfn_valid() and
check memblock for RAM - please don't! :D

> I don't think we can rely on that to eliminate all non-RAM reserved
> pages from the mem_map and avoid having to initialize them in the
> first place. Some could remain as in this case since in the same
> pageblock there's non-RAM followed by RAM and all pfn are valid.
> 
>> In any case, compaction code can't fix this with better range checks.
> 
> David's correct that it can, by adding enough PageReserved (I'm
> running all systems reproducing this with plenty of PageReserved
> checks in all places to work around it until we do a proper fix).
> 
> My problem with that is that 1) it's simply non enforceable at runtime
> that there is not missing PageReserved check and 2) what benefit it
> would provide to leave a wrong zoneid in reserved pages and having to
> add extra PageReserved checks?

See my other mail. If we have a clean way to set *any* memmap (non-RAM,
memory holes at any place) to a proper nid/zid, then we won't need
reserved checks. I raised some cases that need more thought than a
simple "hole in zone".


-- 
Thanks,

David / dhildenb