Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Mar 11, 2009 at 11:46:17AM -0700, Linus Torvalds wrote:
> 
> 
> On Wed, 11 Mar 2009, Andrea Arcangeli wrote:
> 
> > On Wed, Mar 11, 2009 at 10:58:17AM -0700, Linus Torvalds wrote:
> > > As far as I can tell, it's the same old problem that we've always had: if 
> > > you fork(), it's unclear who is going to do the first write - parent or 
> > > child (and "parent" in this case can include any number of threads that 
> > > share the VM, of course).
> > 
> > The child doesn't touch any page. Calling fork just generates O_DIRECT
> > corruption in the parent regardless of what the child does.
> 
> You aren't listening.
> 
> It depends on who does the write. If the _parent_ does the write (with 
> another thread or not), then the _parent_ gets the COW.
> 
> That's all I said.

I only wanted to clarify this doesn't require the child to touch the
page at all.

> If the idiots who use O_DIRECT don't understand that, then hey, it's their 
> problem. I have long been of the opinion that we should not support 
> O_DIRECT at all, and that it's a totally broken premise to start with. 

Well if you don't like it used by databases, O_DIRECT is still ideal for
KVM. Guest caches runs at cpu core speed unlike host cache. Not that
KVM can reproduce this bug (all ram where KVM would be doing O_DIRECT
is mapped MADV_DONTFORK, and besides guest physical ram has to be
allocated with memalign(4096) ;).

Said that I agree it'd be better off to nuke O_DIRECT than to leave
this bug as O_DIRECT should not break the usual memory-protection
semantics provided by read() and fork() syscalls.

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