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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 17 Jan 2023 13:28:27 -0800
Message-ID: <CAJuCfpHawn-hj3yK6MEMHsvnH9xgO87h7Sj3_g0FA7rgut_mVg@mail.gmail.com>
Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it
To:     Jann Horn <jannh@google.com>
Cc:     peterz@infradead.org, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will@kernel.org>, akpm@linux-foundation.org,
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On Tue, Jan 17, 2023 at 10:03 AM Jann Horn <jannh@google.com> wrote:
>
> +locking maintainers

Thanks! I'll CC the locking maintainers in the next posting.

>
> On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > Introduce a per-VMA rw_semaphore to be used during page fault handling
> > instead of mmap_lock. Because there are cases when multiple VMAs need
> > to be exclusively locked during VMA tree modifications, instead of the
> > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> > exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> > mmap_write_lock holder is done with all modifications and drops mmap_lock,
> > it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> > locked.
> [...]
> > +static inline void vma_read_unlock(struct vm_area_struct *vma)
> > +{
> > +       up_read(&vma->lock);
> > +}
>
> One thing that might be gnarly here is that I think you might not be
> allowed to use up_read() to fully release ownership of an object -
> from what I remember, I think that up_read() (unlike something like
> spin_unlock()) can access the lock object after it's already been
> acquired by someone else. So if you want to protect against concurrent
> deletion, this might have to be something like:
>
> rcu_read_lock(); /* keeps vma alive */
> up_read(&vma->lock);
> rcu_read_unlock();

But for deleting VMA one would need to write-lock the vma->lock first,
which I assume can't happen until this up_read() is complete. Is that
assumption wrong?

>
> But I'm not entirely sure about that, the locking folks might know better.
>
> Also, it might not matter given that the rw_semaphore part is removed
> in the current patch 41/41 anyway...

This does matter because Michal suggested dropping that last 41/41
patch for now.