linux-kernel - Re: [RFC PATCH v2 01/10] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE

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Message-ID: <20250430004703.63rumj4znewlbc2h@master>
Date: Wed, 30 Apr 2025 00:47:03 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/10] mm/mremap: introduce more mergeable mremap
 via MREMAP_RELOCATE_ANON

On Tue, Apr 22, 2025 at 09:09:20AM +0100, Lorenzo Stoakes wrote:
[...]
>+bool vma_had_uncowed_children(struct vm_area_struct *vma)
>+{
>+	struct anon_vma *anon_vma = vma ? vma->anon_vma : NULL;
>+	bool ret;
>+
>+	if (!anon_vma)
>+		return false;
>+
>+	/*
>+	 * If we're mmap locked then there's no way for this count to change, as
>+	 * any such change would require this lock not be held.
>+	 */
>+	if (rwsem_is_locked(&vma->vm_mm->mmap_lock))
>+		return anon_vma->num_children > 1;

Hi, Lorenzo

May I have a question here?

>+
>+	/*
>+	 * Any change that would increase the number of children would be
>+	 * prevented by a read lock.
>+	 */
>+	anon_vma_lock_read(anon_vma);
>+	ret = anon_vma->num_children > 1;
>+	anon_vma_unlock_read(anon_vma);
>+
>+	return ret;
>+}

-- 
Wei Yang
Help you, Help me