Re: [patch 2/4] vfs: add set_page_dirty_notag

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-02-17 at 10:38 +0100, Nick Piggin wrote:

> It is a great shame that filesystems are not properly notified
> that a page may become dirty before the actual set_page_dirty
> event (which is not allowed to fail and is called after the
> page is already dirty).

Not quite true, for example the set_page_dirty() done by the write fault
code is done _before_ the page becomes dirty.

This before/after thing was the reason for that horrid file corruption
bug that dragged on for a few weeks back in .19 (IIRC).

> This is a big problem I have with fsblock simply in trying to
> make the memory allocation robust. page_mkwrite unfortunately
> is racy and I've fixed problems there... the big problem though
> is get_user_pages. Fixing that properly seems to require fixing
> callers so it is not really realistic in the short term.

Right, I'm just not sure what we can do, even with a
prepage_page_dirty() function, what are you going to do, fail the fault?