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Date: Mon, 15 Apr 2024 17:13:36 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Peter Xu <peterx@redhat.com>,
	"Liam R. Howlett" <Liam.Howlett@oracle.com>,
	LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks
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On Mon, Apr 15, 2024 at 08:58:28AM -0700, Suren Baghdasaryan wrote:
> On Fri, Apr 12, 2024 at 8:19 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, Apr 12, 2024 at 09:53:29AM -0500, Suren Baghdasaryan wrote:
> > > Unless vmf_anon_prepare() already explains why vma->anon_vma poses a
> > > problem for per-vma locks, we should have an explanation there. This
> > > comment would serve that purpose IMO.
> >
> > I'll do you one better; here's some nice kernel-doc for
> > vmd_anon_prepare():
> 
> I was looking at the find_tcp_vma(), which seems to be the only other
> place where lock_vma_under_rcu() is currently used. I think it's used
> there only for file-backed pages, so I don't think your change affects
> that usecase but this makes me think that we should have some kind of
> a warning for lock_vma_under_rcu() future users... Maybe your addition
> of mmap_assert_locked() inside __anon_vma_prepare() is enough. Please
> don't forget to include that assertion into your final patch.

That's patch 1/3 on the git branch I pointed you to.

The tcp vma is not file backed, but I'm pretty sure that COW is not
something they want, so there's never an anon_vma.  It's for pages
that contain received TCP packets; ie it's mmaped TCP.