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 14:15:20 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        jannh@...gle.com, willy@...radead.org, 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

* Linus Torvalds <torvalds@...ux-foundation.org> [230803 14:02]:
> 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.
> 

I thought the same thing initially, but Suren pointed out that it's not
necessary to hold the vma lock to allocate a vma object.  And it seems
there is at least one user (arch/ia64/mm/init.c) which does allocate
outside the lock during ia64_init_addr_space(), which is fine but I'm
not sure it gains much to do it this way - the insert needs to take the
lock anyways and it is hardly going to be contended.

Anywhere else besides an address space setup would probably introduce a
race.

Thanks,
Liam