Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH] AFS: Implement shared-writable mmap [try #2] | Date | Wed, 16 May 2007 17:12:08 +0100 |
| |
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |