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Date:   Wed, 6 Jun 2018 18:47:23 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>, mhocko@kernel.org,
        vbabka@suse.cz, Aaron Lu <aaron.lu@intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mremap: Increase LATENCY_LIMIT of mremap to reduce the
 number of TLB shootdowns
Message-ID: <20180606174723.bag3o55fvqp6nbvc@techsingularity.net>
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On Wed, Jun 06, 2018 at 08:55:15AM -0700, Nadav Amit wrote:
> > -#define LATENCY_LIMIT	(64 * PAGE_SIZE)
> > +#define LATENCY_LIMIT	(PMD_SIZE)
> > 
> > unsigned long move_page_tables(struct vm_area_struct *vma,
> > 		unsigned long old_addr, struct vm_area_struct *new_vma,
> 
> This LATENCY_LIMIT is only used in move_page_tables() in the following
> manner:
> 
>   next = (new_addr + PMD_SIZE) & PMD_MASK;
>   if (extent > next - new_addr)
>       extent = next - new_addr;
>   if (extent > LATENCY_LIMIT)
>       extent = LATENCY_LIMIT;
>    
> If LATENCY_LIMIT is to be changed to PMD_SIZE, then IIUC the last condition
> is not required, and LATENCY_LIMIT can just be removed (assuming there is no
> underflow case that hides somewhere).
> 

I see no problem removing it other than we may forget that we ever limited
PTE lock hold times for any reason. I'm skeptical it will matter unless
mremap-intensive workloads are a lot more common than I believe.

-- 
Mel Gorman
SUSE Labs