Re: [PATCH v3 2/2] mm: disable CONFIG_PER_VMA_LOCK until its fixed

[David Hildenbrand]

On 05.07.23 22:25, Peter Xu wrote:
> On Wed, Jul 05, 2023 at 10:22:27AM -0700, Suren Baghdasaryan wrote:
> > On Wed, Jul 5, 2023 at 10:16 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > On 05.07.23 19:12, Suren Baghdasaryan wrote:
> > > > A memory corruption was reported in [1] with bisection pointing to the
> > > > patch [2] enabling per-VMA locks for x86.
> > > > Disable per-VMA locks config to prevent this issue while the problem is
> > > > being investigated. This is expected to be a temporary measure.
> > > >
> > > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=217624
> > > > [2] https://lore.kernel.org/all/20230227173632.3292573-30-surenb@xxxxxxxxxx
> > > >
> > > > Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
> > > > Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@xxxxxxxxxx/
> > > > Reported-by: Jacob Young <jacobly.alt@xxxxxxxxx>
> > > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624
> > > > Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > > ---
> > > >    mm/Kconfig | 3 ++-
> > > >    1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/Kconfig b/mm/Kconfig
> > > > index 09130434e30d..0abc6c71dd89 100644
> > > > --- a/mm/Kconfig
> > > > +++ b/mm/Kconfig
> > > > @@ -1224,8 +1224,9 @@ config ARCH_SUPPORTS_PER_VMA_LOCK
> > > >           def_bool n
> > > >
> > > >    config PER_VMA_LOCK
> > > > -     def_bool y
> > > > +     bool "Enable per-vma locking during page fault handling."
> > > >        depends on ARCH_SUPPORTS_PER_VMA_LOCK && MMU && SMP
> > > > +     depends on BROKEN
> > > >        help
> > > >          Allow per-vma locking during page fault handling.
> > > Do we have any testing results (that don't reveal other issues :) ) for
> > > patch #1? Not sure if we really want to mark it broken if patch #1 fixes
> > > the issue.
> >
> > I tested the fix using the only reproducer provided in the reports
> > plus kernel compilation and my fork stress test. All looked good and
> > stable but I don't know if other reports had the same issue or
> > something different.
>
> The commit log seems slightly confusing.  It mostly says the bug was still
> not solved, but I assume patch 1 is the current "fix", it's just not clear
> whether there's any other potential issues?
>
> According to the stable tree rules:
>
>   - It must fix a problem that causes a build error (but not for things
>     marked CONFIG_BROKEN), an oops, a hang, data corruption, a real
>     security issue, or some "oh, that's not good" issue.  In short, something
>     critical.
>
> I think it means vma lock will never be fixed in 6.4, and it can't (because
> after this patch it'll be BROKEN, and this patch copies stable, and we
> can't fix BROKEN things in stables).
>
> Totally no problem I see, just to make sure this is what you wanted..
>
> There'll still try to be a final fix, am I right?  As IIRC allowing page
> faults during fork() is one of the major goals of vma lock.

At least not that I am aware of (and people who care about that should 
really work on scalable fork() alternatives, like that io_uring fork() 
thingy).

My understanding is that CONFIG_PER_VMA_LOCK wants to speed up page 
concurrent page faults *after* fork() [or rather, after new process 
creation], IOW, when we have a lot of mmap() activity going on while 
some threads of the new process are already active and don't actually 
touch what's getting newly mmaped.

--
Cheers,

David / dhildenb