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Date: Fri, 12 Apr 2024 14:32:48 +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>,
	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 Fri, Apr 12, 2024 at 05:46:52AM -0700, Suren Baghdasaryan wrote:
> On Thu, Apr 11, 2024 at 8:14 PM Matthew Wilcox <willy@infradead.org> wrote:
> About the code, I'll take a closer look once I'm back from vacation
> this weekend but I think you will also have to modify
> do_anonymous_page() to use vmf_anon_prepare() instead of
> anon_vma_prepare().

Ah yes.  Also do_huge_pmd_anonymous_page().  And we should do this:

+++ b/mm/rmap.c
@@ -182,8 +182,6 @@ static void anon_vma_chain_link(struct vm_area_struct *vma,
  * for the new allocation. At the same time, we do not want
  * to do any locking for the common case of already having
  * an anon_vma.
- *
- * This must be called with the mmap_lock held for reading.
  */
 int __anon_vma_prepare(struct vm_area_struct *vma)
 {
@@ -191,6 +189,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
        struct anon_vma *anon_vma, *allocated;
        struct anon_vma_chain *avc;

+       mmap_assert_locked(mm);
        might_sleep();

        avc = anon_vma_chain_alloc(GFP_KERNEL);

> > We could even eagerly initialise vma->anon_vma for anon vmas.  I don't
> > know why we don't do that.
> 
> You found the answer to that question a long time ago and IIRC it was
> because in many cases we end up not needing to set vma->anon_vma at
> all. So, this is an optimization to try avoiding extra operations
> whenever we can. I'll try to find your comment on this.

I thought that was file VMAs that I found the answer to that question?