Re: [PATCH v4 12/17] mm/huge_memory: move anon_vma handling into the anon split helper

[Kairui Song]

On Tue, Sep 8, 2026 at 11:56 PM Kiryl Shutsemau <kas@xxxxxxxxxx> wrote:
>
> On Tue, Sep 08, 2026 at 02:12:16AM +0800, Kairui Song via B4 Relay wrote:
> > +     /*
> > +      * Unmap/remap needs the anon_vma. The caller does not necessarily
> > +      * hold an mmap_lock that would prevent the anon_vma from
> > +      * disappearing, so we first take a reference and lock it.
> > +      *
> > +      * An unmapped folio needs none of this: folio_get_anon_vma() and
> > +      * folio_lock_anon_vma_read() both bail out on !folio_mapped()
> > +      * before taking the lock, and folio_ref_freeze() below still
> > +      * rejects a folio that picked up a reference meanwhile. Note
> > +      * a swapped-out THP counts as unmapped here as swap PTEs do
> > +      * not contribute mapcount, and they are splittable.
> > +      */
> >       if (folio_mapped(folio)) {
> > -             need_remap = true;
> > +             anon_vma = folio_get_anon_vma(folio);
> > +             if (!anon_vma)
> > +                     return -EBUSY;
> > +             anon_vma_lock_write(anon_vma);
> >               ret = unmap_folio(folio);
> >               if (ret)
> > -                     return ret;
> > +                     goto out_unlock;
> >       }
>
> Hm. Nothing serializes folio_mapped() here. I believe it is fine, but the
> reasoning deserves a comment, since it is what the whole change rests on.
> Something along the lines of:
>
>        * folio_mapped() is not stable here, but it can only change in
>        * one direction while the folio is locked. The mapcount can drop
>        * to zero at any time, zap_pte_range() takes no folio lock. It
>        * cannot go up: swapin, migration and uffd move all lock the folio
>        * before mapping it, and fork only copies PTEs that already exist.
>        *
>        * So if we see the folio mapped, the worst case is an empty rmap
>        * walk. If we see it unmapped, it stays unmapped. Anything else
>        * needs a reference first and folio_ref_freeze() below catches it.

Thanks for the review and suggestion!

Sure, let me update the comment, Zi also asked about comment update.