linux-kernel - Re: [Cluster-devel] [PATCH 06/32] sched: Add task_struct->faults_disabled

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Message-ID: <ZHBwEYupK2gxLOjn@moria.home.lan>
Date:   Fri, 26 May 2023 04:38:41 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>, Jan Kara <jack@...e.cz>,
        cluster-devel@...hat.com, "Darrick J . Wong" <djwong@...nel.org>,
        linux-kernel@...r.kernel.org, dhowells@...hat.com,
        linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: [Cluster-devel] [PATCH 06/32] sched: Add
 task_struct->faults_disabled_mapping

On Fri, May 26, 2023 at 01:10:43AM -0700, Christoph Hellwig wrote:
> On Thu, May 25, 2023 at 07:20:46PM -0400, Kent Overstreet wrote:
> > > > I'm absolutely not in favour to add workarounds for thes kind of locking
> > > > problems to the core kernel.  I already feel bad for allowing the
> > > > small workaround in iomap for btrfs, as just fixing the locking back
> > > > then would have avoid massive ratholing.
> > > 
> > > Please let me know when those btrfs changes are in a presentable shape ...
> > 
> > I would also be curious to know what btrfs needs and what the approach
> > is there.
> 
> btrfs has the extent locked, where "extent locked" is a somewhat magic
> range lock that actually includes different lock bits.  It does so
> because it clears the page writeback bit when the data made it to the
> media, but before the metadata required to find it is commited, and
> the extent lock prevents it from trying to do a readpage on something
> that has actually very recently been written back but not fully
> commited.  Once btrfs is changed to only clear the page writeback bit
> once the write is fully commited like in other file systems this extra
> level of locking can go away, and there are no more locks in the
> readpage path that are also taken by the direct I/O code.  With that
> a lot of code in btrfs working around this can go away, including the
> no fault direct I/O code.

wow, yeah, that is not how that is supposed to work...