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

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Message-ID: <921a7a3f-8552-4eb0-880b-9dac822ed256@suse.cz>
Date: Wed, 30 Apr 2025 14:50:27 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Wei Yang <richard.weiyang@...il.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 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 4/30/25 02:47, Wei Yang wrote:
> 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?

You're missing the actual question :)

>>+
>>+	/*
>>+	 * 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;
>>+}
>