linux-kernel - Re: [RFC][Patch v11 1/2] mm: page

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Date:   Mon, 15 Jul 2019 11:33:01 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        pbonzini@...hat.com, lcapitulino@...hat.com, pagupta@...hat.com,
        wei.w.wang@...el.com, yang.zhang.wz@...il.com, riel@...riel.com,
        mst@...hat.com, dodgen@...gle.com, konrad.wilk@...cle.com,
        dhildenb@...hat.com, aarcange@...hat.com,
        alexander.duyck@...il.com, john.starks@...rosoft.com,
        mhocko@...e.com
Subject: Re: [RFC][Patch v11 1/2] mm: page_hinting: core infrastructure

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