Re: [PATCH 6/7] xfs: use iomap_valid method to detect stale cached iomaps

[Christoph Hellwig <hch@infradead.org>]

On Thu, Nov 03, 2022 at 08:39:27AM +1100, Dave Chinner wrote:
> My concerns with putting it into the iomap is that different
> filesystems will have different mechanisms for detecting stale
> iomaps. THe way we do it with a generation counter is pretty coarse
> as any change to the extent map will invalidate the iomap regardless
> of whether they overlap or not.

OTOH it is a good way to nudge users to at least implement this simple
but working scheme, and it has no real overhead if people later want
to do something more fancy.

> If, in future, we want something more complex and finer grained
> (e.g. an iext tree cursor) to allow us to determine if the
> change to the extent tree actually modified the extent backing the
> iomap, then we are going to need an opaque cookie of some kind, not
> a u32 or a u32*.

Yes, but for that it actually has to be worth it and be implemented.