linux-kernel - Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking

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Date:   Sat, 8 Jul 2023 15:36:15 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     akpm@...ux-foundation.org, regressions@...mhuis.info,
        bagasdotme@...il.com, jacobly.alt@...il.com, willy@...radead.org,
        liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
        ldufour@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        regressions@...ts.linux.dev, Jiri Slaby <jirislaby@...nel.org>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking

On Sat, Jul 8, 2023 at 2:18 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sat, 8 Jul 2023 at 12:12, Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> >  kernel/fork.c | 1 +
> >  1 file changed, 1 insertion(+)
>
> I ended up editing your explanation a lot.
>
> I'm not convinced that the bug has much to do with the delayed tlb flushing.
>
> I think it's more fundamental than some tlb coherence issue: our VM
> copying simply expects to not have any unrelated concurrent page fault
> activity, and various random internal data structures simply rely on
> that.
>
> I made up an example that I'm not sure is relevant to any of the
> particular failures, but that I think is a non-TLB case: the parent
> 'vma->anon_vma' chain is copied by dup_mmap() in anon_vma_fork(), and
> it's possible that the parent vma didn't have any anon_vma associated
> with it at that point.
>
> But a concurrent page fault to the same vma - even *before* the page
> tables have been copied, and when the TLB is still entirely coherent -
> could then cause a anon_vma_prepare() on that parent vma, and
> associate one of the pages with that anon-vma.
>
> Then the page table copy happens, and that page gets marked read-only
> again, and is added to both the parent and the child vma's, but the
> child vma never got associated with the parents new anon_vma, because
> it didn't exist when anon_vma_fork() happened.
>
> Does this ever happen? I have no idea. But it would seem to be an
> example that really has nothing to do with any TLB state, and is just
> simply "we cannot handle concurrent page faults while we're busy
> copying the mm".
>
> Again - maybe I messed up, but it really feels like the missing
> vma_start_write() was more fundamental, and not some "TLB coherency"
> issue.

Sounds plausible. I'll try to use the reproducer to verify if that's
indeed happening here. It's likely there are multiple problematic
scenarios due to this missing lock though.
Thanks,
Suren.

>
>             Linus