linux-kernel - Re: [PATCH 1/2] mm: introduce put_user

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Date:   Wed, 12 Dec 2018 16:18:33 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jérôme Glisse <jglisse@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <john.hubbard@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, tom@...pey.com,
        Al Viro <viro@...iv.linux.org.uk>, benve@...co.com,
        Christoph Hellwig <hch@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        rcampbell@...dia.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

On Wed, Dec 12, 2018 at 4:01 PM Jerome Glisse <jglisse@...hat.com> wrote:
>
> On Wed, Dec 12, 2018 at 04:37:03PM -0700, Jason Gunthorpe wrote:
> > On Wed, Dec 12, 2018 at 04:53:49PM -0500, Jerome Glisse wrote:
> > > > Almost, we need some safety around assuming that DMA is complete the
> > > > page, so the notification would need to go all to way to userspace
> > > > with something like a file lease notification. It would also need to
> > > > be backstopped by an IOMMU in the case where the hardware does not /
> > > > can not stop in-flight DMA.
> > >
> > > You can always reprogram the hardware right away it will redirect
> > > any dma to the crappy page.
> >
> > That causes silent data corruption for RDMA users - we can't do that.
> >
> > The only way out for current hardware is to forcibly terminate the
> > RDMA activity somehow (and I'm not even sure this is possible, at
> > least it would be driver specific)
> >
> > Even the IOMMU idea probably doesn't work, I doubt all current
> > hardware can handle a PCI-E error TLP properly.
>
> What i saying is reprogram hardware to crappy page ie valid page
> dma map but that just has random content as a last resort to allow
> filesystem to reuse block. So their should be no PCIE error unless
> hardware freak out to see its page table reprogram randomly.

Hardware has a hard enough time stopping I/O to existing page let
alone switching to a new one in the middle of a transaction. This is a
non-starter, but it's also a non-concern because the bulk of DMA is
transient. For non-transient DMA there is a usually a registration
phase where the capability to support revocation can be validated,