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SubjectRe: Test12 ll_rw_block error.

On Thu, 14 Dec 2000, Linus Torvalds wrote:

> Good point.
> This actually looks fairly nasty to fix. The obvious fix would be to not
> put such buffers on the dirty list at all, and instead rely on the VM
> layer calling "writepage()" when it wants to push out the pages.
> That would be the nice behaviour from a VM standpoint.
> However, that assumes that you don't have any "anonymous" buffers, which
> is probably an unrealistic assumption.
> The problem is that we don't have any per-buffer "writebuffer()" function,
> the way we have them per-page. It was never needed for any of the normal
> filesystems, and XFS just happened to be able to take advantage of the
> b_end_io behaviour.
> Suggestions welcome.

Just one: any fs that really cares about completion callback is very likely
to be picky about the requests ordering. So sync_buffers() is very unlikely
to be useful anyway.

In that sense we really don't have anonymous buffers here. I seriously
suspect that "unrealistic" assumption is not unrealistic at all. I'm
not sufficiently familiar with XFS code to say for sure, but...

What we really need is a way for VFS/VM to pass the pressure on filesystem.
That's it. If fs wants unusual completions for requests - let it have its
own queueing mechanism and submit these requests when it finds that convenient.

Stephen, you probably already thought about that area. Could you comment on

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