On Fri, 9 Feb 2007 11:32:58 +0100 Nick Piggin <[email protected]> wrote:
> On Fri, Feb 09, 2007 at 02:09:54AM -0800, Andrew Morton wrote:
> > On Fri, 9 Feb 2007 10:54:05 +0100 Nick Piggin <[email protected]> wrote:
> >
> > >
> > > That's still got a deadlock,
> >
> > It does?
>
> Yes, PG_lock vs mm->mmap_sem.
Where? It requires that someone hold mmap_sem for writing as well as a
page lock (in an order which would require some thought). Do we ever do
that?
> > > and also it doesn't work if we want to lock
> > > the page when performing a minor fault (which I want to fix fault vs
> > > invalidate),
> >
> > It's hard to discuss this without a description of what you want to fix
> > there, and a description of how you plan to fix it.
>
> http://marc.theaimsgroup.com/?l=linux-mm&m=116865911432667&w=2
mutter.
Could perhaps fix that by running ClearPageUptodate in invalidate, thus
forcing the pagefault code to take the page lock (which we already hold).
That does mean that we'll fleetingly have a non-uptodate page in pagetables
which is a bit nasty.
Or, probably better, we could add a new page flag (heh) telling nopage that
it needs to lock the page even if it's uptodate.
> > > and also assumes nobody's ->nopage locks the page or
> > > requires any resources that are held by prepare_write (something not
> > > immediately clear to me with the cluster filesystems, at least).
> >
> > The nopage handler is filemap_nopage(). Are there any exceptions to that?
>
> OCFS2 and GFS2.
So the rule is that ->nopage handlers against pagecache mustn't lock the
page if it's already uptodate. That's OK. But it might conflict with the
above invalidate proposal.
Gad. ocfs2_nopage() diddles with signals.
> > > But that all becomes legacy path, so do we really care? Supposing fs
> > > maintainers like perform_write, then after the main ones have implementations
> > > we could switch over to the slow-but-correct prepare_write legacy path.
> > > Or we could leave it, or we could use Linus's slightly-less-buggy scheme...
> > > by that point I expect I'd be sick of arguing about it ;)
> >
> > It's worth "arguing" about. This is write(). What matters more??
>
> That's the legacy path that uses prepare/commit (ie. supposing that all
> major filesystems did get converted to perform_write).
We'll never, ever, ever update and test all filesytems. What you're
calling "legacy" code will be there for all time.
I haven't had time to look at the perform_write stuff yet.
> Of course I would still want my correct-but-slow version in that case,
> but I just wouldn't care to argue if you still wanted to keep it fast.
This is write(). We just cannot go and double-copy all the memory or take
mmap_sem and do a full pagetable walk in there. It just means that we
haven't found a suitable solution yet.
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