linux-kernel - Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <59c47ed5-a565-4220-823c-a278130092d5@lucifer.local>
Date:   Mon, 15 May 2023 12:31:51 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
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        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
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        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        "Paul E . McKenney" <paulmck@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On Sun, May 14, 2023 at 10:14:46PM -0700, Christoph Hellwig wrote:
> On Sun, May 14, 2023 at 08:20:04PM +0100, Lorenzo Stoakes wrote:
> > As discussed at LSF/MM, on the flight over I wrote a little repro [0] which
> > reliably triggers the ext4 warning by recreating the scenario described
> > above, using a small userland program and kernel module.
> >
> > This code is not perfect (plane code :) but does seem to do the job
> > adequately, also obviously this should only be run in a VM environment
> > where data loss is acceptable (in my case a small qemu instance).
>
> It would be really awesome if you could wire it up with and submit it
> to xfstests.

Sure am happy to take a look at that! Also happy if David finds it useful in any
way for this unit tests.

The kernel module interface is a bit sketchy (it takes a user address which it
blindly pins for you) so it's not something that should be run in any unsafe
environment but as long as we are ok with that :)