Re: [PATCH] AFS: Implement shared-writable mmap [try #2]

[David Howells <dhowells@redhat.com>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> In general (modulo bugs and crazy filesystems), you're not allowed to have
> !uptodate pages mapped into user addresses because that implies the user
> would be allowed to see garbage.

Ths situation I have to deal with is a tricky one.  Consider:

 (1) User A modifies a page with his key.  This change gets made in the
     pagecache, but is not written back immediately.

 (2) User B then wants to modify the same page, but with a different key.
     This means that afs_prepare_write() has to flush A's writes back to the
     server before B is permitted to write.

 (3) The flush fails because A is no longer permitted to write to that file.
     This means that the change in the page cache is now stale.  We can't just
     write it back as B because B didn't make the change.

What I've made afs_prepare_write() do in this situation is to nuke A's entire
write.  We can't write any of it back.  I can't call invalidate_inode_pages()
or similar because that might incorrectly kill one of B's writes (or someone
else's writes); besides, the on-server file hasn't changed.

To nuke A's write, each page that makes up that write is marked non-uptodate
and then reloaded.  Whilst I might wish to call invalidate_inode_pages_range(),
I can't as it can/would deadlock if called from prepare_write() in two
different ways.

> >>Minor issue: you can just check for `if (!page->mapping)` for truncation,
> >>which is the usual signal to tell the reader you're checking for truncate.
> >
> >
> > That's inconsistent with other core code, truncate_complete_page() for
> > example.
> 
> Your filesystem internally moves pages between mappings like tmpfs?

You misunderstand me.  truncate_complete_page() uses this:

	if (page->mapping != mapping)

not this:

	if (!page->mapping)

I think that both cases should work in page_mkwrite().  But !page->mapping does
not appear to be the "usual signal" from what I've seen.

However, that's a minor matter.

David