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

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Message-ID: <20250430154119.a5ljf5t5tutqzim5@master>
Date: Wed, 30 Apr 2025 15:41:19 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Wei Yang <richard.weiyang@...il.com>,
	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 Wed, Apr 30, 2025 at 02:15:24PM +0100, Lorenzo Stoakes wrote:
>On Wed, Apr 30, 2025 at 12:47:03AM +0000, 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?
>
>Just ask the question.
>

Thanks.

My question is the function is expected to return true, if we have forked a
vma from this one, right?

IMO there are cases when it has one forked child and anon_vma->num_children == 1,
which means folios are not exclusively mapped. But the function would return
false.

Or maybe I misunderstand the logic here.

>However, with respect, the last drive-by review you gave was not helpful,
>so I strongly suggest that this is not a great use of your time.
>
>Again, I _strongly_ suggest you focus on bug fixes or the like.

Thanks for your suggestion and patience. I would try to focus on bugs and skip
those subtle things.

>
>Thanks.
>
>>
>> >+
>> >+	/*
>> >+	 * 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

-- 
Wei Yang
Help you, Help me