Re: [PATCH 3/4] xfs: validate writeback mapping using data fork seq counter

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jan 17, 2019 at 11:35:17AM -0500, Brian Foster wrote:
> Hmm, it would be nice if these fixes were separate from the whole
> always_cow thing. Some initial thoughts on a quick look through the
> first few patches on the v3 post:

We can always skip the last patch.  It just helps to really nicely
show a lot of the problems that are otherwise hard to reproduce, but
already exist.

FYI, I just resent it like a minute before reading your mail.

> 1. It's probably best to drop your xfs_trim_extent_eof() changes as I
> have a stable patch to add a couple more calls and then I subsequently
> remove the whole thing going forward. Refactoring it is just churn at
> this point.

Sure.

> 2. The whole explicit race with truncate detection looks rather involved
> to me at first glance. I'm trying to avoid relying on i_size at all for
> this because it doesn't seem like a reliable approach. E.g., Dave
> described a hole punch vector for the same fundamental problem this
> series is trying to address:
> 
>   https://marc.info/?l=linux-xfs&m=154692641021480&w=2
> 
> I don't think looking at i_size really helps us with that, but I could
> be missing other changes in the cow series.

The i_size detection isn't new in this series, just slightly moved
around.  And it really is just intended as an optimization to not
even bother if we are beyond i_size.

> 
> In general I'm looking at putting something like this in
> xfs_iomap_write_allocate() once the data fork sequence number tracking
> is enabled:
> 
>                         /*
>                          * Now that we have ILOCK we must account for the fact
>                          * that the fork (and thus our mapping) could have
>                          * changed while the inode was unlocked. If the fork
>                          * has changed, trim the caller's mapping to the
>                          * current extent in the fork.

We don't even look at the callers mapping except for the range to
cover.  And that is how e.g. direct I/O also works and a good thing
as far as I can tell.  To make use of the previous mapping we'd have
to rewrite xfs_bmapi_write.

If we want to be able to reuse existing mapings I think the sequences
are helping us a bit, but a lot more work is needed, and it should
be done in a generic way and not just in this path.