From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200])
	by kanga.kvack.org (Postfix) with ESMTP id 26C606B000D
	for <linux-mm@kvack.org>; Tue,  9 Oct 2018 17:39:02 -0400 (EDT)
Received: by mail-pg1-f200.google.com with SMTP id w15-v6so2356537pge.2
        for <linux-mm@kvack.org>; Tue, 09 Oct 2018 14:39:02 -0700 (PDT)
Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12])
        by mx.google.com with ESMTPS id b15-v6si25312187plk.356.2018.10.09.14.39.00
        for <linux-mm@kvack.org>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Tue, 09 Oct 2018 14:39:00 -0700 (PDT)
Date: Tue, 9 Oct 2018 14:38:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: Speed up mremap on large regions
Message-Id: <20181009143859.8b9a700b1caf4e8d1e33a723@linux-foundation.org>
In-Reply-To: <20181009201400.168705-1-joel@joelfernandes.org>
References: <20181009201400.168705-1-joel@joelfernandes.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@android.com, minchan@google.com, hughd@google.com, lokeshgidra@google.com, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Kate Stewart <kstewart@linuxfoundation.org>, Philippe Ombredanne <pombredanne@nexb.com>, Thomas Gleixner <tglx@linutronix.de>

On Tue,  9 Oct 2018 13:14:00 -0700 "Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:

> Android needs to mremap large regions of memory during memory management
> related operations. The mremap system call can be really slow if THP is
> not enabled. The bottleneck is move_page_tables, which is copying each
> pte at a time, and can be really slow across a large map. Turning on THP
> may not be a viable option, and is not for us. This patch speeds up the
> performance for non-THP system by copying at the PMD level when possible.
> 
> The speed up is three orders of magnitude. On a 1GB mremap, the mremap
> completion times drops from 160-250 millesconds to 380-400 microseconds.
> 
> Before:
> Total mremap time for 1GB data: 242321014 nanoseconds.
> Total mremap time for 1GB data: 196842467 nanoseconds.
> Total mremap time for 1GB data: 167051162 nanoseconds.
> 
> After:
> Total mremap time for 1GB data: 385781 nanoseconds.
> Total mremap time for 1GB data: 388959 nanoseconds.
> Total mremap time for 1GB data: 402813 nanoseconds.
> 
> Incase THP is enabled, the optimization is skipped. I also flush the
> tlb every time we do this optimization since I couldn't find a way to
> determine if the low-level PTEs are dirty. It is seen that the cost of
> doing so is not much compared the improvement, on both x86-64 and arm64.

Looks tasty.

> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -191,6 +191,54 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
>  		drop_rmap_locks(vma);
>  }
>  
> +bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,

I'll park this for now, shall plan to add a `static' in there then
merge it up after 4.20-rc1.