Re: [PATCH v3] mm: use per_vma lock for MADV_DONTNEED

[Lorenzo Stoakes]

On Sat, Jun 07, 2025 at 04:53:02PM +1200, Barry Song wrote:
> On Sat, Jun 7, 2025 at 4:05 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Jun 07, 2025 at 12:46:23PM +1200, Barry Song wrote:
> > > To simplify handling, the implementation falls back to the standard
> > > mmap_lock if userfaultfd is enabled on the VMA, avoiding the complexity of
> > > userfaultfd_remove().
> >
> > This feels too complex to me.  Why do we defer grabbing the vma lock
> > so late, instead of grabbing it at the start like the fault handler does?
>
> Hi Matthew,
>
> It looks like you missed the spot where your comment should have gone:
> https://lore.kernel.org/all/0b96ce61-a52c-4036-b5b6-5c50783db51f@lucifer.local/
> So I believe Lorenzo is the best person to respond to your concern.
>
> In both v1 and v2 [1][2], we did try to fall back as early as possible:
>
> [1] https://lore.kernel.org/linux-mm/20250527044145.13153-1-21cnbao@xxxxxxxxx/
> [2] https://lore.kernel.org/all/20250530104439.64841-1-21cnbao@xxxxxxxxx/
>
> But that approach had its own problems:
> * It's not extensible to other madvise operations.
> * It's not easy to adapt to vector_madvise.
>
> I also initially found the new approach too complex and tried a few
> alternatives, but each had its own problems. In the end, Lorenzo's
> solution still seems to be the cleanest among them.
>
> I even forgot to move the code below back to visit() from
> madvise_vma_behavior(). I had changed it while exploring an
> alternative and should have reverted it.
>
> +       if (madv_behavior && madv_behavior->lock_mode ==
> MADVISE_VMA_READ_LOCK) {
> +               vma = try_vma_read_lock(mm, madv_behavior, start, end);
> +               if (vma) {
> +                       error = madvise_vma_behavior(vma, &prev, start, end,
> +                               madv_behavior);  /* better to be visit() */
> +                       vma_end_read(vma);
> +                       return error;
> +               }
> +       }

Ah damn missed that :) and I just tagged haha, yeah that should be visit().

I hate this pattern, maybe will refactor in future... entirely for this rather
hacked in anon_vma_name implementation :(

>
> Thanks
> Barry