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Message-ID: <CAJuCfpHsdU5e7+f29ezDXus5-W7jT580YCY0LpErTfKJjeo8zQ@mail.gmail.com>
Date: Fri, 1 Aug 2025 08:30:45 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, jannh@...gle.com, Liam.Howlett@...cle.com,
lorenzo.stoakes@...cle.com, pfalcato@...e.de, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: change vma_start_read() to drop RCU lock on failure
On Fri, Aug 1, 2025 at 2:09 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 7/31/25 17:19, Suren Baghdasaryan wrote:
> > vma_start_read() can drop and reacquire RCU lock in certain failure
> > cases. It's not apparent that the RCU session started by the caller of
> > this function might be interrupted when vma_start_read() fails to lock
> > the vma. This might become a source of subtle bugs and to prevent that
> > we change the locking rules for vma_start_read() to drop RCU read lock
> > upon failure. This way it's more obvious that RCU-protected objects are
> > unsafe after vma locking fails.
> >
> > Suggested-by: Vlastimil Babka <vbabka@...e.cz>
>
> IIRC you considered it yourself, I just convinced you to try :)
>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
>
> I thought we didn't need the drop rcu lock for -EAGAIN, but that would just
> made it more complex for little gain, so this looks good to me.
Yes, we technically don't but this way it's simpler to explain (drop
RCU on any failure).
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> Nit:
>
> > @@ -223,11 +227,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > MA_STATE(mas, &mm->mm_mt, address, address);
> > struct vm_area_struct *vma;
> >
> > - rcu_read_lock();
> > retry:
> > + rcu_read_lock();
> > vma = mas_walk(&mas);
> > - if (!vma)
> > + if (!vma) {
> > + rcu_read_unlock();
> > goto inval;
> > + }
> >
> > vma = vma_start_read(mm, vma);
> > if (IS_ERR_OR_NULL(vma)) {
> > @@ -241,6 +247,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > /* Failed to lock the VMA */
> > goto inval;
> > }
> > +
> > + rcu_read_unlock();
>
> Would it make sense to put this under the comment below?
>
> > +
> > /*
> > * At this point, we have a stable reference to a VMA: The VMA is
> > * locked and we know it hasn't already been isolated.
>
> Give it continues like this:
>
> * From here on, we can access the VMA without worrying about which
>
> * fields are accessible for RCU readers.
Yep, will change.
Thanks!
>
> > @@ -249,16 +258,14 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> > */
> >
> > /* Check if the vma we locked is the right one. */
> > - if (unlikely(address < vma->vm_start || address >= vma->vm_end))
> > - goto inval_end_read;
> > + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> > + vma_end_read(vma);
> > + goto inval;
> > + }
> >
> > - rcu_read_unlock();
> > return vma;
> >
> > -inval_end_read:
> > - vma_end_read(vma);
> > inval:
> > - rcu_read_unlock();
> > count_vm_vma_lock_event(VMA_LOCK_ABORT);
> > return NULL;
> > }
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