linux-kernel - Re: [linus:master] [gup] a425ac5365: WARNING:at_mm/gup.c:#__get_user

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Date:   Wed, 5 Jul 2023 10:15:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
        lkp@...el.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [linus:master] [gup] a425ac5365: WARNING:at_mm/gup.c:#__get_user_pages

On Wed, 5 Jul 2023 at 08:54, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I'll tighten it up, and switch the WARN_ON_ONCE() to just do a
> "dump_stack()" so that it won't cause problems with the syzbot tests
> either.

I pushed it out. It's based on the thing that Oliver already tested,
but expanded a bit from that (and with a better calling convention, so
that when we're done we can just delete the whole helper function and
replace it with "find_vma()" like it is supposed to just be).

It's lightly tested, but I no longer have any trivial ways to trigger
the warning, so the testing was literally "now it doesn't say anything
at all".

It should still trigger the warning by literally doing some direct-IO
GUP below the stack - but at least the obvious cases of false
positives hopefully don't trigger.

At least until somebody comes up with another obvious case that I
didn't think of ;)

               Linus