linux-kernel - Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <20250113093611.hmjmizvt73mrq2y2@master>
Date: Mon, 13 Jan 2025 09:36:11 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
	liam.howlett@...cle.com, lorenzo.stoakes@...cle.com,
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Subject: Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a
 reference count

On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote:
[...]
> 
>+static inline bool is_vma_writer_only(int refcnt)
>+{
>+	/*
>+	 * With a writer and no readers, refcnt is VMA_LOCK_OFFSET if the vma
>+	 * is detached and (VMA_LOCK_OFFSET + 1) if it is attached. Waiting on
>+	 * a detached vma happens only in vma_mark_detached() and is a rare
>+	 * case, therefore most of the time there will be no unnecessary wakeup.
>+	 */
>+	return refcnt & VMA_LOCK_OFFSET && refcnt <= VMA_LOCK_OFFSET + 1;

It looks equivalent to 

	return (refcnt == VMA_LOCK_OFFSET) || (refcnt == VMA_LOCK_OFFSET + 1);

And its generated code looks a little simpler.

>+}
>+
>+static inline void vma_refcount_put(struct vm_area_struct *vma)
>+{
>+	/* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
>+	struct mm_struct *mm = vma->vm_mm;
>+	int oldcnt;
>+
>+	rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
>+	if (!__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt)) {
>+
>+		if (is_vma_writer_only(oldcnt - 1))
>+			rcuwait_wake_up(&mm->vma_writer_wait);
>+	}
>+}
>+

-- 
Wei Yang
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