linux-kernel - Re: [PATCH v3 1/9] mm: add find_vma()-like API but RCU protected and taking VMA lock

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Date: Wed, 5 Jun 2024 09:33:11 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-fsdevel@...r.kernel.org,
        brauner@...nel.org, viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        gregkh@...uxfoundation.org, linux-mm@...ck.org, surenb@...gle.com,
        rppt@...nel.org
Subject: Re: [PATCH v3 1/9] mm: add find_vma()-like API but RCU protected and
 taking VMA lock

* Matthew Wilcox <willy@...radead.org> [240604 20:57]:
> On Tue, Jun 04, 2024 at 05:24:46PM -0700, Andrii Nakryiko wrote:
> > +/*
> > + * find_and_lock_vma_rcu() - Find and lock the VMA for a given address, or the
> > + * next VMA. Search is done under RCU protection, without taking or assuming
> > + * mmap_lock. Returned VMA is guaranteed to be stable and not isolated.
> 
> You know this is supposed to be the _short_ description, right?
> Three lines is way too long.  The full description goes between the
> arguments and the Return: line.
> 
> > + * @mm: The mm_struct to check
> > + * @addr: The address
> > + *
> > + * Returns: The VMA associated with addr, or the next VMA.
> > + * May return %NULL in the case of no VMA at addr or above.
> > + * If the VMA is being modified and can't be locked, -EBUSY is returned.
> > + */
> > +struct vm_area_struct *find_and_lock_vma_rcu(struct mm_struct *mm,
> > +					     unsigned long address)
> > +{
> > +	MA_STATE(mas, &mm->mm_mt, address, address);
> > +	struct vm_area_struct *vma;
> > +	int err;
> > +
> > +	rcu_read_lock();
> > +retry:
> > +	vma = mas_find(&mas, ULONG_MAX);
> > +	if (!vma) {
> > +		err = 0; /* no VMA, return NULL */
> > +		goto inval;
> > +	}
> > +
> > +	if (!vma_start_read(vma)) {
> > +		err = -EBUSY;
> > +		goto inval;
> > +	}
> > +
> > +	/*
> > +	 * Check since vm_start/vm_end might change before we lock the VMA.
> > +	 * Note, unlike lock_vma_under_rcu() we are searching for VMA covering
> > +	 * address or the next one, so we only make sure VMA wasn't updated to
> > +	 * end before the address.
> > +	 */
> > +	if (unlikely(vma->vm_end <= address)) {
> > +		err = -EBUSY;
> > +		goto inval_end_read;
> > +	}
> > +
> > +	/* Check if the VMA got isolated after we found it */
> > +	if (vma->detached) {
> > +		vma_end_read(vma);
> > +		count_vm_vma_lock_event(VMA_LOCK_MISS);
> > +		/* The area was replaced with another one */
> 
> Surely you need to mas_reset() before you goto retry?

Probably more than that.  We've found and may have adjusted the
index/last; we should reconfigure the maple state.  You should probably
use mas_set(), which will reset the maple state and set the index and
long to address.


> 
> > +		goto retry;
> > +	}
>