Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

[Suren Baghdasaryan]

On Fri, Apr 12, 2024 at 6:32 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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@infradeadorg> 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);
>

Yes.

> > > 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?

I'll try to find that discussion once I get back to my workstation this weekend.