From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 727C93A8F8 for ; Fri, 1 Mar 2024 15:01:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709305291; cv=none; b=PmrktXH6I9HEhkaogDs0ZdIs+i9Q1R2OF8WjlOgNbZ/urjzW3eDM8oN56aY7UeWuI4ygwtudoKOmsBIBAoEeqTyO4v5PjcuGxqASL+DDhttwgtdoJO3yA0lP7BYa9lqKDlmiy2STn6fs+iX4YQMS70Z4XD1pXryTEttjCUGRdlE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709305291; c=relaxed/simple; bh=ERUK+MDr7h76tICDSai6O14g7fu2rTNwoDp0AUWnhMw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=E6L6oGD0cLkdAI69fb62v6wzY8nMe+0YBM+3qRlW7DBLy/+w5pCTvZaO2ZUPiFI4WuW/99nhqJPv5hNAmnml4Mj4MyhcjreXufxmcgFSBYeoB2h/dgbpkDdchgBUxVhJYG3tAI/wgDDhh/1bsMb4jr6QO68rlMmDnW8gp3Lti1Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=BzKOx16t; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="BzKOx16t" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1709305288; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=h2DPueeqFYkRZUwqD/vr2uzl6MwQXTsD7wTc6ZLo2F0=; b=BzKOx16tABweugfHWSAE9iK7bjL6hnTqxBv3oDsHdzKUEM670Km6+9ONvwrHesJNBaoxZO 8ii+DbFNQeROJ6d1Y2FQbHb0W6q1kjvcN5BpNbIQvPzC3NbdoUHyaNW/qgHO0D7wNl3FjF Qq+hGT+sTHpTrr/XRJ8wJu8nZH9LJVQ= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-92-srloWFL3MSietjrkSqTZhg-1; Fri, 01 Mar 2024 10:01:25 -0500 X-MC-Unique: srloWFL3MSietjrkSqTZhg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7488583B82C; Fri, 1 Mar 2024 15:01:24 +0000 (UTC) Received: from bfoster (unknown [10.22.32.137]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 37D0E1C060AF; Fri, 1 Mar 2024 15:01:24 +0000 (UTC) Date: Fri, 1 Mar 2024 10:03:06 -0500 From: Brian Foster To: Kent Overstreet Cc: linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org, djwong@kernel.org Subject: Re: [PATCH 01/21] bcachefs: KEY_TYPE_accounting Message-ID: References: <20240225023826.2413565-1-kent.overstreet@linux.dev> <20240225023826.2413565-2-kent.overstreet@linux.dev> Precedence: bulk X-Mailing-List: linux-bcachefs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.7 On Thu, Feb 29, 2024 at 04:24:37PM -0500, Kent Overstreet wrote: > On Thu, Feb 29, 2024 at 01:43:15PM -0500, Brian Foster wrote: > > Hmm.. I think the connection I missed on first look is basically > > disk_accounting_key_to_bpos(). I think what is confusing is that calling > > this a key makes me think of bkey, which I understand to contain a bpos, > > so then overlaying it with a bpos didn't really make a lot of sense to > > me conceptually. > > > > So when I look at disk_accounting_key_to_bpos(), I see we are actually > > using the bpos _pad field, and this structure basically _is_ the bpos > > for a disk accounting btree bkey. So that kind of makes me wonder why > > this isn't called something like disk_accounting_pos instead of _key, > > but maybe that is wrong for other reasons. > > hmm, I didn't consider calling it disk_accounting_pos. I'll let that > roll around in my brain. > > 'key' is more standard terminology to me outside bcachefs, but 'pos' > does make more sense within bcachefs. > Ok, so I'm not totally crazy at least. :) Note again that wasn't an explicit suggestion, just that it seems more logical to me based on my current understanding. I'm just trying to put down my initial thoughts/confusions in hopes that at least some of this triggers ideas for improvements... > > Either way, what I'm trying to get at is that I think this documentation > > would be better if it explained conceptually how disk_accounting_key > > relates to bkey/bpos, and why it exists separately from bkey vs. other > > key types, rather than (or at least before) getting into the lower level > > side effects of a union with bpos. > > Well, that gets into some fun territory - ideally bpos would not be a > fixed thing that every btree was forced to use, we'd be able to define > different types per btree. > Ok, but this starts to sound orthogonal to the accounting bits. Since I don't really grok why this is called a key, here's how I would add to the existing documentation: "Here, the key has considerably more structure than a typical key (bpos); an accounting key is 'struct disk_accounting_key', which is a union of bpos. We do this because disk_account_key actually is bpos for the related bkey that ends up in the accounting btree. This btree uses nontraditional bpos semantics because accounting btree keys are indexed differently . Yadda yadda.. Unlike with other key types, ... " Hm? Brian > And we're actually going to need to be able to do that in order to do > configurationless autotiering - i.e. tracking how hot/cold data is on an > inode:offset basis, because LRU btree backreferences need to go in the > key (bpos), not the value, in order to avoid collisions, and bpos isn't > big enough for that. > > disk_accounting_(key|pos) is an even trickier situation, because of > endianness issues. The trick we do with bpos of defining the field order > differently based on endianness so that byte order matches word order - > that really wouldn't work here, so there is at present no practical way > that I know of to avoid the byte swabbing when going back and forth > between bpos and disk_accounting_pos on big endian. > > But gcc does have an attribute now that lets you specify that an integer > struct member is big or little endian... I if we could get them to go > one step further and give us an attribute to control whether members are > laid out in ascending or descending order... >