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Date:   Mon, 24 Apr 2023 14:49:35 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Dave Chinner <david@fromorbit.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
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        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
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        Jakub Kicinski <kuba@kernel.org>,
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        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
 default
Message-ID: <ZEbBLzy4SU8IZR68@ziepe.ca>
References: <f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com>
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On Sun, Apr 23, 2023 at 11:56:48PM +0100, Lorenzo Stoakes wrote:

> This warned upon check should in reality not occur, because it implies the
> GUP user is trying to do something broken and is _not_ explicitly telling
> GUP that it knows it's doing it and can live with the consequences. And on
> that basis, is worthy of a warning so we know we have to go put this flag
> in that place (and know it is a source of problematic GUP usage), or fix
> the caller.

It is fine for debugging, but we can't merge user triggerable
WARN_ONs..

Since the GUP caller has no idea if userspace might be maliciously
passing in a file VMA we can't throw warnings.

Jason