linux-kernel - Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it

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Message-ID: <CAG48ez0RhQ6=W01brLUXDXqQxz2M1FEMNqd2OvL+LhcJQY=NqA@mail.gmail.com>
Date:   Tue, 17 Jan 2023 19:02:55 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Suren Baghdasaryan <surenb@...gle.com>, peterz@...radead.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com,
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        gthelen@...gle.com, gurua@...gle.com, arjunroy@...gle.com,
        soheil@...gle.com, hughlynch@...gle.com, leewalsh@...gle.com,
        posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it

+locking maintainers

On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan <surenb@...gle.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 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...