On Tue, Feb 6, 2024 at 6:35 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
>
> * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240205 17:24]:
> > On Mon, Feb 5, 2024 at 2:00 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote:
> > >
> > > * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240205 16:55]:
> > > ...
> > >
> > > > > > > We can take care of anon_vma as well here right? I can take a bool
> > > > > > > parameter ('prepare_anon' or something) and then:
> > > > > > >
> > > > > > > if (vma) {
> > > > > > > if (prepare_anon && vma_is_anonymous(vma)) &&
> > > > > > > !anon_vma_prepare(vma)) {
> > > > > > > vma = ERR_PTR(-ENOMEM);
> > > > > > > goto out_unlock;
> > > > > > > }
> > > > > > > > vma_aquire_read_lock(vma);
> > > > > > > }
> > > > > > > out_unlock:
> > > > > > > > mmap_read_unlock(mm);
> > > > > > > > return vma;
> > > > > > > > }
> > > > > >
> > > > > > Do you need this? I didn't think this was happening in the code as
> > > > > > written? If you need it I would suggest making it happen always and
> > > > > > ditch the flag until a user needs this variant, but document what's
> > > > > > going on in here or even have a better name.
> > > > >
> > > > > I think yes, you do need this. I can see calls to anon_vma_prepare()
> > > > > under mmap_read_lock() protection in both mfill_atomic_hugetlb() and
> > > > > in mfill_atomic(). This means, just like in the pagefault path, we
> > > > > modify vma->anon_vma under mmap_read_lock protection which guarantees
> > > > > that adjacent VMAs won't change. This is important because
> > > > > __anon_vma_prepare() uses find_mergeable_anon_vma() that needs the
> > > > > neighboring VMAs to be stable. Per-VMA lock guarantees stability of
> > > > > the VMA we locked but not of its neighbors, therefore holding per-VMA
> > > > > lock while calling anon_vma_prepare() is not enough. The solution
> > > > > Lokesh suggests would call anon_vma_prepare() under mmap_read_lock and
> > > > > therefore would avoid the issue.
> > > > >
> > >
> > > ...
> > >
> > > > anon_vma_prepare() is also called in validate_move_areas() via move_pages().
> > >
> > > Probably worth doing it unconditionally and have a comment as to why it
> > > is necessary.
> > >
> > The src_vma (in case of move_pages()) doesn't need to have it.
> >
> > The only reason I'm not inclined to make it unconditional is what if
> > some future user of lock_vma() doesn't need it for their purpose? Why
> > allocate anon_vma in that case.
>
> Because there isn't a user and it'll add a flag that's a constant. If
> there is a need for the flag later then it can be added at that time.
> Maybe there will never be a user and we've just complicated the code for
> no reason. Don't implement features that aren't necessary, especially
> if there is no intent to use them.
>
I'm not too attached to the idea of keeping it conditional. But I have
already sent v3 which currently does it conditionally. Please take a
look at it. Along with any other comments/changes that I get, I'll
also make it unconditional in v4, if you say so.
> >
> > > Does this avoid your locking workaround?
> >
> > Not sure which workaround you are referring to. I am almost done
> > implementing your suggestion. Very soon will share the next version of
> > the patch-set.
>
> The locking dance with the flags indicating if it's per-vma lock or
> mmap_lock.
>
That dance was not because of anon_vma. It's just that I hadn't
realized that we can do it the way you suggested :) I really liked
your suggestion and is implemented in v3. PTAL.
