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

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Message-ID: <Zl-38XrUw9entlFR@casper.infradead.org>
Date: Wed, 5 Jun 2024 01:57:21 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: 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,
	liam.howlett@...cle.com, 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

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?

> +		goto retry;
> +	}