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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 3 Aug 2023 11:01:51 -0700
Message-ID: <CAHk-=wiCrWAoEesBuoGoqqufvesicbGp3cX0LyKgEvsFaZNpDA@mail.gmail.com>
Subject: Re: [PATCH v3 5/6] mm: always lock new vma before inserting into vma tree
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, jannh@google.com, willy@infradead.org,
        liam.howlett@oracle.com, david@redhat.com, peterx@redhat.com,
        ldufour@linux.ibm.com, vbabka@suse.cz, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, hannes@cmpxchg.org,
        dave@stgolabs.net, hughd@google.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, stable@vger.kernel.org
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On Thu, 3 Aug 2023 at 10:27, Suren Baghdasaryan <surenb@google.com> wrote:
>
> While it's not strictly necessary to lock a newly created vma before
> adding it into the vma tree (as long as no further changes are performed
> to it), it seems like a good policy to lock it and prevent accidental
> changes after it becomes visible to the page faults. Lock the vma before
> adding it into the vma tree.

So my main reaction here is that I started to wonder about the vma allocation.

Why doesn't vma_init() do something like

        mmap_assert_write_locked(mm);
        vma->vm_lock_seq = mm->mm_lock_seq;

and instead we seem to expect vma_lock_alloc() to do this (and do it
very badly indeed).

Strange.

Anyway, this observation was just a reaction to that "not strictly
necessary to lock a newly created vma" part of the commentary. I feel
like we could/should just make sure that all newly created vma's are
always simply created write-locked.

                Linus