Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem

["Theodore Y. Ts'o" <tytso@mit.edu>]

(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