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From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, yebin <yebin10@huawei.com>,
	linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	Mark Fasheh <mark@fasheh.com>, Joel Becker <jlbec@evilplan.org>,
	Joseph Qi <joseph.qi@linux.alibaba.com>
Subject: Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem
Date: Tue, 25 Aug 2020 10:10:20 -0400	[thread overview]
Message-ID: <20200825141020.GA668551@mit.edu> (raw)
In-Reply-To: <20200825121616.GA10294@infradead.org>

(Adding the OCFS2 maintainers, since my possibly insane idea proposed
below would definitely impact them!)

On Tue, Aug 25, 2020 at 01:16:16PM +0100, Christoph Hellwig wrote:
> On Tue, Aug 25, 2020 at 02:05:54PM +0200, Jan Kara wrote:
> > Discarding blocks and buffers under a mounted filesystem is hardly
> > anything admin wants to do. Usually it will confuse the filesystem and
> > sometimes the loss of buffer_head state (including b_private field) can
> > even cause crashes like:
> 
> Doesn't work if the file system uses multiple devices.  I think we
> just really need to split the fs buffer_head address space from the
> block device one.  Everything else is just going to cause a huge mess.

I wonder if we should go a step further, and stop using struct
buffer_head altogether in jbd2 and ext4 (as well as ocfs2).

This would involve moving whatever structure elements from the
buffer_head struct into journal_head, and manage writeback and reads
requests directly in jbd2.  This would allow us to get detailed write
errors back, which is currently not possible from the buffer_head
infrastructure.

The downside is this would be a pretty massive change in terms of LOC,
since we use struct buffer_head in a *huge* number of places.  If
we're careful, most of it could be handled by a Coccinelle script to
rename "struct buffer_head" to "struct journal_head".  Fortunately, we
don't actually use that much of the fs/buffer_head functions in
fs/{ext4,ocfs2}/*.c.

One potentially tricky bit is that ocfs2 hasn't been converted to
using iomap, so it's still using __blockdev_direct_IO.  So it's data
blocks for DIO would still have to use struct buffer_head (which means
the Coccinelle script won't really work for fs/ocfs2, without a lot of
manual rework) --- or ocfs2 would have to switched to use iomap at
least for DIO support.

What do folks think?

						- Ted


  reply	other threads:[~2020-08-25 14:11 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-25 12:05 [PATCH RFC 0/2] Block and buffer invalidation under a filesystem Jan Kara
2020-08-25 12:05 ` [PATCH RFC 1/2] fs: Don't invalidate page buffers in block_write_full_page() Jan Kara
2020-08-25 12:05 ` [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem Jan Kara
2020-08-25 12:16   ` Christoph Hellwig
2020-08-25 14:10     ` Theodore Y. Ts'o [this message]
2020-08-25 15:12       ` Jan Kara
2020-08-25 18:41         ` Matthew Wilcox
2020-08-25 18:49           ` Jan Kara
2020-08-25 14:50     ` Jan Kara
2020-08-27  7:16       ` Christoph Hellwig
2020-08-27 21:39         ` Al Viro
2020-08-28  0:07           ` Dave Chinner
2020-08-28  8:10             ` Jan Kara
2020-08-28  8:21     ` Andreas Dilger
2020-08-29  6:40       ` Christoph Hellwig
2020-08-31  7:48         ` Jan Kara

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