David Howells wrote:
Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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.
Why would that kill anyone's writes?
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.
Which ways? Are you talking about prepare_write being called from page_mkwrite,
or anywhere?
More generally it sounds like a nasty thing to have a writeback cache if it can
become incoherent (due to dirty pages that subsequently cannot be written
back) without notification. Have you tried doing a write-through one?
You may be clearing PG_uptodate, but isn't there still an underlying problem
that you can have mappings to the page at that point? If that isn't a problem
for you, then I don't know why you would have to clear PG_uptodate at all.
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.
truncate_complete_page does that because it has to handle the case where
the mapping changes from one thing to something else that is non-NULL,
which tmpfs does.
This is not the case for most code in fs.
--
SUSE Labs, Novell Inc.
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