Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

[Peter Xu]

On Thu, Sep 24, 2020 at 03:39:53PM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 24, 2020 at 02:34:18PM -0400, Peter Xu wrote:
> 
> > > >        // RDONLY gup
> > > >        pin_user_pages(buf, !WRITE);
> > > >        // pte of buf duplicated on both sides
> > > >        fork();
> > > >        mprotect(buf, WRITE);
> > > >        *buf = 1;
> > > >        // buf page replaced as cow triggered
> > > > 
> > > > Currently when fork() we'll happily share a pinned read-only page with the
> > > > child by copying the pte directly.  
> > > 
> > > Why? This series prevents that, the page will be maybe_dma_pinned, so
> > > fork() will copy it.
> > 
> > With the extra mprotect(!WRITE), I think we'll see a !pte_write() entry.  Then
> > it'll not go into maybe_dma_pinned() at all since cow==false.
> 
> Hum that seems like a problem in this patch, we still need to do the
> DMA pinned logic even if the pte is already write protected.

Yes I agree.  I'll take care of that in the next version too.

-- 
Peter Xu