linux-kernel - Re: [PATCH v2 4/6] mm: lock vma explicitly before doing vm_flags_reset and vm_flags_reset

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Message-ID: <CAHk-=wix_+xyyAXf+02Pgt3xEpfKncjT8A6n1Oa+9uKH8bXnEA@mail.gmail.com>
Date:   Wed, 2 Aug 2023 10:49:35 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, jannh@...gle.com, willy@...radead.org,
        liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
        ldufour@...ux.ibm.com, vbabka@...e.cz, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, hannes@...xchg.org,
        dave@...olabs.net, hughd@...gle.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 4/6] mm: lock vma explicitly before doing
 vm_flags_reset and vm_flags_reset_once

On Tue, 1 Aug 2023 at 15:07, Suren Baghdasaryan <surenb@...gle.com> wrote:
>
>       To make locking more visible, change these
> functions to assert that the vma write lock is taken and explicitly lock
> the vma beforehand.

So I obviously think this is a good change, but the fact that it
touched driver files makes me go "we're still doing something wrong".

I'm not super-happy with hfi1_file_mmap() doing something like
vma_start_write(), in that I *really* don't think drivers should ever
have to think about issues like this.

And I think it's unnecessary.  This is the mmap op in the
hfi1_file_ops, and I think that any actual mmap() code had _better_
had locked the new vma before asking any driver to set things up (and
the assert would catch it if somebody didn't).

I realize that it doesn't hurt in a technical sense, but I think
having drivers call these VM-internal subtle locking functions does
hurt in a maintenance sense, so we should make sure to not have it.

                   Linus