Re: Why doesn't zap_pte_range() call page_mkwrite()

[Nick Piggin <npiggin@suse.de>]

On Tue, Sep 08, 2009 at 11:30:07AM -0400, Chris Mason wrote:
> > > As I said, I think I can fix the NFS problem by simply unmapping the
> > > page inside ->writepage() whenever we know the write request was
> > > originally set up by a page fault.
> > 
> > The biggest outstanding problem we have remaining is get_user_pages.
> > Callers are only required to hold a ref on the page and then they
> > can call set_page_dirty at any point after that.
> > 
> > I have a half-done patch somewhere to add a put_user_pages, and then
> > we could probably go from there to pinning the fs metadata (whether
> > by using the page lock or something else, I don't quite know).
> 
> Hi everyone,
> 
> Sorry for digging up an old thread, but is there any reason we can't
> just use page_mkwrite here?  I'd love to get rid of the btrfs code to
> detect places that use set_page_dirty without a page_mkwrite.

It is because page_mkwrite must be called before the page is dirtied
(it may fail, it theoretically may do something crazy with the previous
clean page data). And in several places I think it gets called from a
nasty context.

It hasn't fallen completely off my radar. fsblock has the same issue
(although I've just been ignoring gup writes into fsblock fs for the
time being).

I have a basic idea of what to do... It would be nice to change calling
convention of get_user_pages and take the page lock. Database people might
scream, in which case we could only take the page lock for filesystems that
define ->page_mkwrite (so shared mem segments avoid the overhead). Lock
ordering might get a bit interesting, but if we can have callers ensure they
always submit and release partially fulfilled requirests, then we can always
trylock them.