linux-ext4 - Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular spinlock

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Message-ID: <20191118093544.GA17319@quack2.suse.cz>
Date:   Mon, 18 Nov 2019 10:35:44 +0100
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, Sebastian Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Matthew Wilcox <willy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.com>, Mark Fasheh <mark@...heh.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Christoph Hellwig <hch@...radead.org>,
        Joel Becker <jlbec@...lplan.org>
Subject: Re: [PATCH] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a regular
 spinlock_t

On Fri 15-11-19 12:36:34, Theodore Y. Ts'o wrote:
> On Fri, Nov 15, 2019 at 03:56:38PM +0100, Jan Kara wrote:
> > With some effort, we could even shrink struct buffer_head from 104 bytes
> > (on x86_64) to 96 bytes but I don't think that effort is worth it (I'd find
> > it better use of time to actually work on getting rid of buffer heads
> > completely).
> 
> Is that really realistic?  All aside from the very large number of
> file systems which use buffer_heads that would have to be reworked,
> the concept of buffer heads is pretty fundamental to how jbd2 is
> architected.

I think it is reasonably possible to remove buffer_heads from data path
(including direct IO path) of all filesystems. That way memory consumption
of buffer_heads becomes mostly irrelevant and we can have a look how much
from the current bh framework still makes sense...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR