Re: Interaction between Xen and XFS: stray RW mappings

Jeremy Fitzhardinge
2007-10-14T23:12:30Z

David Chinner wrote:
> You mean xfs_buf.c.
>

Yes, sorry.

> And yes, we delay unmapping pages until we have a batch of them
> to unmap. vmap and vunmap do not scale, so this is batching helps
> alleviate some of the worst of the problems.
>

How much performance does it cost? What kind of workloads would it show
up under?

> Realistically, if this delayed release of vmaps is a problem for
> Xen, then I think that some generic VM solution is needed to this
> problem as vmap() is likely to become more common in future (think
> large blocks in filesystems). Nick - any comments?
>

Well, the only real problem is that the pages are returned to the free
pool and reallocated while still being part of a mapping. If the pages
are still owned by the filesystem/pagecache, then there's no problem.

What's the lifetime of things being vmapped/unmapped in xfs? Are they
necessarily being freed when they're unmapped, or could unmapping of
freed memory be more immediate than other memory?

Maybe it just needs a notifier chain or something.

J