linux-kernel - Re: [PATCH v3] mm: use per_vma lock for MADV

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Message-ID: <aEO6asiCu9oG1z8o@casper.infradead.org>
Date: Sat, 7 Jun 2025 05:04:58 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Lokesh Gidra <lokeshgidra@...gle.com>,
	Tangquan Zheng <zhengtangquan@...o.com>,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v3] mm: use per_vma lock for MADV_DONTNEED

On Sat, Jun 07, 2025 at 12:46:23PM +1200, Barry Song wrote:
> To simplify handling, the implementation falls back to the standard
> mmap_lock if userfaultfd is enabled on the VMA, avoiding the complexity of
> userfaultfd_remove().

This feels too complex to me.  Why do we defer grabbing the vma lock
so late, instead of grabbing it at the start like the fault handler does?