linux-kernel - Re: [PATCH v13 08/14] mm: multi-gen LRU: support page table walks

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Date:   Thu, 7 Jul 2022 01:23:55 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Yu Zhao <yuzhao@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "page-reclaim@...gle.com" <page-reclaim@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte 
        <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v13 08/14] mm: multi-gen LRU: support page table walks

* Yu Zhao <yuzhao@...gle.com> [220706 18:26]:
> On Wed, Jul 06, 2022 at 04:00:17PM -0600, Yu Zhao wrote:
> 
> ...
> 
> > +/*
> > + * Some userspace memory allocators map many single-page VMAs. Instead of
> > + * returning back to the PGD table for each of such VMAs, finish an entire PMD
> > + * table to reduce zigzags and improve cache performance.
> > + */
> > +static bool get_next_vma(unsigned long mask, unsigned long size, struct mm_walk *args,
> > +			 unsigned long *vm_start, unsigned long *vm_end)
> > +{
> > +	unsigned long start = round_up(*vm_end, size);
> > +	unsigned long end = (start | ~mask) + 1;
> > +
> > +	VM_WARN_ON_ONCE(mask & size);
> > +	VM_WARN_ON_ONCE((start & mask) != (*vm_start & mask));
> > +
> > +	while (args->vma) {
> > +		if (start >= args->vma->vm_end) {
> > +			args->vma = args->vma->vm_next;
> > +			continue;
> > +		}
> > +
> > +		if (end && end <= args->vma->vm_start)
> > +			return false;
> > +
> > +		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args)) {
> > +			args->vma = args->vma->vm_next;
> > +			continue;
> > +		}
> > +
> > +		*vm_start = max(start, args->vma->vm_start);
> > +		*vm_end = min(end - 1, args->vma->vm_end - 1) + 1;
> > +
> > +		return true;
> > +	}
> > +
> > +	return false;
> > +}
> 
> To make the above work on top of the Maple Tree:
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7096ff7836db..c0c1195da803 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3689,23 +3689,14 @@ static bool get_next_vma(unsigned long mask, unsigned long size, struct mm_walk
>  {
>  	unsigned long start = round_up(*vm_end, size);
>  	unsigned long end = (start | ~mask) + 1;
> +	VMA_ITERATOR(vmi, args->mm, start);
>  
>  	VM_WARN_ON_ONCE(mask & size);
>  	VM_WARN_ON_ONCE((start & mask) != (*vm_start & mask));
>  
> -	while (args->vma) {
> -		if (start >= args->vma->vm_end) {
> -			args->vma = args->vma->vm_next;
> +	for_each_vma_range(vmi, args->vma, end) {
> +		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args))
>  			continue;
> -		}
> -
> -		if (end && end <= args->vma->vm_start)
> -			return false;
> -
> -		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args)) {
> -			args->vma = args->vma->vm_next;
> -			continue;
> -		}
>  
>  		*vm_start = max(start, args->vma->vm_start);
>  		*vm_end = min(end - 1, args->vma->vm_end - 1) + 1;

This looks correct to me.  There are a few subtle things that happen
with this change but it all works out rather nicely.


Thanks,
Liam