linux-kernel - Re: [syzbot] [mm?] WARNING in __gup_longterm

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Message-ID: <CAHk-=whzthQy42SzYb1Bs_6tGyss5=SoiOppSE6onjUWDwA=aw@mail.gmail.com>
Date:   Tue, 4 Jul 2023 09:39:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     syzbot <syzbot+6cf44e127903fdf9d929@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in __gup_longterm_locked

On Tue, 4 Jul 2023 at 09:24, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> Thanks.  This is the temporary warning which was added by Linus's
> a425ac5365f6cb3cc4 ("gup: add warning if some caller would seem to want
> stack expansion").

Yes, and the randomizer system calls aren't very interesting for that warning.

I don't have any good idea for how to distinguish "this is a
randomizer that is just doing crazy things by its very nature and is
passing in nonsensical system call arguments" from "this is a real
application that is doing crazy things that we will sadly have to try
to be backwards compatible with".

And at the same time, I _really_ don't want that warning to then
perhaps hide some *other* more real warning from the test automation.

End result: I'd love for that warning to trigger on real applications
(including ones run by any cloud test infrastructure, although I doubt
that infrastructure necessarily runs very interesting loads), but not
on things like syzbot and trinity that just randomize system calls.

Does anybody have any ideas how to tell them apart? Maybe syzbot
already sets some flag for this purpose that I just haven't thought
of?

                 Linus