linux-kernel - Re: [PATCH] mm: Always sanity check anon

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Date: Sat, 13 Apr 2024 23:52:30 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Peter Xu <peterx@...hat.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lokesh Gidra <lokeshgidra@...gle.com>,
	Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

On Sat, Apr 13, 2024 at 02:46:56PM -0700, Suren Baghdasaryan wrote:
> On Fri, Apr 12, 2024 at 8:31 AM Matthew Wilcox <willy@...radead.org> wrote:
> >  - Rename lock_vma() to uffd_lock_vma() because it really is uffd
> >    specific.
> 
> I'm planning to expand the scope of lock_vma() and reuse it for
> /proc/pid/maps reading under per-VMA locks. No objection to renaming
> it for now but I'll likely rename it back later once it's used in more
> places.

That would seem like a mistake.  The uffd lock_vma() will create an
anon_vma for VMAs that don't have one, and you wouldn't want that.
It seems to me that lock_vma_under_rcu() does everything you want except
the fallback to mmap_read_lock().  And I'm not sure there's a good way
to package that up ... indeed, I don't see why you'd want the "take
the mmap_lock, look up the VMA, drop the mmap read lock" part at all --
once you've got the mmap_lock, just hold it until you're done.