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

Jason Gunthorpe
2020-09-24T18:39:58Z

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.

Jason