linux-kernel - Re: [PATCH v3 0/1] mm: introduce put_user

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Date:   Wed, 13 Mar 2019 19:16:51 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Jerome Glisse <jglisse@...hat.com>
cc:     john.hubbard@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, Al Viro <viro@...iv.linux.org.uk>,
        Christian Benvenuti <benve@...co.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Tom Talpey <tom@...pey.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder
 versions

On Tue, 12 Mar 2019, Jerome Glisse wrote:

> > > This has been discuss extensively already. GUP usage is now widespread in
> > > multiple drivers, removing that would regress userspace ie break existing
> > > application. We all know what the rules for that is.

You are still misstating the issue. In RDMA land GUP is widely used for
anonyous memory and memory based filesystems. *Not* for real filesystems.

> > Because someone was able to get away with weird ways of abusing the system
> > it not an argument that we should continue to allow such things. In fact
> > we have repeatedly ensured that the kernel works reliably by improving the
> > kernel so that a proper failure is occurring.
>
> Driver doing GUP on mmap of regular file is something that seems to
> already have widespread user (in the RDMA devices at least). So they
> are active users and they were never told that what they are doing
> was illegal.

Not true. Again please differentiate the use cases between regular
filesystem and anonyous mappings.

> > Well swapout cannot occur if the page is pinned and those pages are also
> > often mlocked.
>
> I would need to check the swapout code but i believe the write to disk
> can happen before the pin checks happens. I believe the event flow is:
> map read only, allocate swap, write to disk, try to free page which
> checks for pin. So that you could write stale data to disk and the GUP
> going away before you perform the pin checks.

Allocate swap is a separate step that associates a swap entry to an
anonymous page.

> They are other thing to take into account and that need proper page
> dirtying, like soft dirtyness for instance.

RDMA mapped pages are all dirty all the time.

> Well RDMA driver maintainer seems to report that this has been a valid
> and working workload for their users.

No they dont.

Could you please get up to date on the discussion before posting?