From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-fsdevel-owner@vger.kernel.org>
Received: from bombadil.infradead.org ([65.50.211.133]:39917 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750835AbdKJIuj (ORCPT
        <rfc822;linux-fsdevel@vger.kernel.org>);
        Fri, 10 Nov 2017 03:50:39 -0500
Date: Fri, 10 Nov 2017 00:50:35 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: NeilBrown <neilb@suse.com>, Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] Improve fairness when locking the per-superblock
 s_anon list
Message-ID: <20171110085035.GA2304@infradead.org>
References: <151019756744.30101.3832608128627682973.stgit@noble>
 <151019772760.30101.8513274540570798315.stgit@noble>
 <CA+55aFwxFCi5mDEadCf8xxK-LQ-aXUhd1Ox6m51vsXt3uexFpw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFwxFCi5mDEadCf8xxK-LQ-aXUhd1Ox6m51vsXt3uexFpw@mail.gmail.com>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Thu, Nov 09, 2017 at 11:52:48AM -0800, Linus Torvalds wrote:
> Yes, getting rid of s_anon might involve crazy things like "let's just
> walk all the dentries at umount time", but honestly, that sounds
> preferable. Especially if we can just then do something like
> 
>  - set a special flag in the superblock if we ever use __d_obtain_alias()
> 
>  - only scan all the dentries on umount if that flag is set.
> 
> Hmm?

The scan would be extremely painful with our current global dcache.

But then again the global dcache isn't exactly a good idea to start
with.  If we had a per-sb dcache using e.g. the resizable hash table
the networking people seem to be fond off your idea would work really
well.