Re: [openib-general] Re: [PATCH repost] PROT_DONTCOPY: ifiniband uverbs fork support

[Hugh Dickins <hugh@veritas.com>]

On Thu, 11 Aug 2005, Gleb Natapov wrote:
> On Thu, Aug 11, 2005 at 03:04:29PM +0100, Hugh Dickins wrote:
> > On Thu, 11 Aug 2005, Gleb Natapov wrote:
> > > What about the idea that was floating around about new VM flag that will
> > > instruct kernel to copy pages belonging to the vma on fork instead of mark
> > > them as cow?
> > 
> > It's a pretty good idea, and thanks for reminding us of it.
> > 
> > It suffers from the general difficulty with fixes within get_user_pages,
> > that we need down_write(&mm->mmap_sem) to split_vma, and even just to
> > update vm_flags, whereas get_user_pages is entered with down_read.
> > 
> Why do it form get_user_pages? Lets use madvise/mprotect interface.
> Program can mrpotect(VM_COPYONFORK) address range before registering it.

Perhaps.  But then it's more complicated than the VM_DONTCOPY we came from.

It's a good solution to the semantic divergence introduced by VM_DONTCOPY,
but most people seemed unworried by that aspect.

My trouble is that I'm waiting for a magic right solution to appear,
and none has struct me that way so far.

Hugh