linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Message-ID: <20200924183953.GG9916@ziepe.ca>
Date:   Thu, 24 Sep 2020 15:39:53 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Peter Xu <peterx@...hat.com>
Cc:     John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

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