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Date:   Mon, 21 Sep 2020 09:03:01 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Jan Kara <jack@suse.cz>
Cc:     John Hubbard <jhubbard@nvidia.com>, Peter Xu <peterx@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
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On Mon, Sep 21, 2020 at 10:35:05AM +0200, Jan Kara wrote:
> > My thinking is to hit this issue you have to already be doing
> > FOLL_LONGTERM, and if some driver hasn't been properly marked and
> > regresses, the fix is to mark it.
> > 
> > Remember, this use case requires the pin to extend after a system
> > call, past another fork() system call, and still have data-coherence.
> > 
> > IMHO that can only happen in the FOLL_LONGTERM case as it inhernetly
> > means the lifetime of the pin is being controlled by userspace, not by
> > the kernel. Otherwise userspace could not cause new DMA touches after
> > fork.
> 
> I agree that the new aggressive COW behavior is probably causing issues
> only for FOLL_LONGTERM users. That being said it would be nice if even
> ordinary threaded FOLL_PIN users would not have to be that careful about
> fork(2) and possible data loss due to COW - we had certainly reports of
> O_DIRECT IO loosing data due to fork(2) and COW exactly because it is very
> subtle how it behaves... But as I wrote above this is not urgent since that
> problematic behavior exists since the beginning of O_DIRECT IO in Linux.

Yes, I agree - what I was thinking is to do this FOLL_LONGTERM for the
rc and then a small patch to make it wider for the next cycle so it
can test in linux-next for a responsible time period.

Interesting to hear you confirm block has also seen subtle user
problems with this as well.

Jason