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SubjectRe: [RFC][Patch v11 1/2] mm: page_hinting: core infrastructure
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On 11.07.19 20:21, Dave Hansen wrote:
> On 7/10/19 12:51 PM, Nitesh Narayan Lal wrote:
>> +static void bm_set_pfn(struct page *page)
>> +{
>> + struct zone *zone = page_zone(page);
>> + int zone_idx = page_zonenum(page);
>> + unsigned long bitnr = 0;
>> +
>> + lockdep_assert_held(&zone->lock);
>> + bitnr = pfn_to_bit(page, zone_idx);
>> + /*
>> + * TODO: fix possible underflows.
>> + */
>> + if (free_area[zone_idx].bitmap &&
>> + bitnr < free_area[zone_idx].nbits &&
>> + !test_and_set_bit(bitnr, free_area[zone_idx].bitmap))
>> + atomic_inc(&free_area[zone_idx].free_pages);
>> +}
>
> Let's say I have two NUMA nodes, each with ZONE_NORMAL and ZONE_MOVABLE
> and each zone with 1GB of memory:
>
> Node: 0 1
> NORMAL 0->1GB 2->3GB
> MOVABLE 1->2GB 3->4GB
>
> This code will allocate two bitmaps. The ZONE_NORMAL bitmap will
> represent data from 0->3GB and the ZONE_MOVABLE bitmap will represent
> data from 1->4GB. That's the result of this code:
>
>> + if (free_area[zone_idx].base_pfn) {
>> + free_area[zone_idx].base_pfn =
>> + min(free_area[zone_idx].base_pfn,
>> + zone->zone_start_pfn);
>> + free_area[zone_idx].end_pfn =
>> + max(free_area[zone_idx].end_pfn,
>> + zone->zone_start_pfn +
>> + zone->spanned_pages);
>
> But that means that both bitmaps will have space for PFNs in the other
> zone type, which is completely bogus. This is fundamental because the
> data structures are incorrectly built per zone *type* instead of per zone.
>

I don't think it's incorrect, it's just not optimal in all scenarios.
E.g., in you example, this approach would "waste" 2 * 1GB of tracking
data for the wholes (2* 64bytes when using 1 bit for 2MB).

FWIW, this is not a numa-specific thingy. We can have sparse zones
easily on single-numa systems.

Node: 0
NORMAL 0->1GB, 2->3GB
MOVABLE 1->2GB, 3->4GB

So tracking it per zones instead instead of zone type is only one part
of the story.

--

Thanks,

David / dhildenb

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