linux-kernel - Re: [PATCH v3 5/6] mm: always lock new vma before inserting into vma tree

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Date:   Thu, 3 Aug 2023 11:01:51 -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 v3 5/6] mm: always lock new vma before inserting into vma tree

On Thu, 3 Aug 2023 at 10:27, Suren Baghdasaryan <surenb@...gle.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