Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

[Suren Baghdasaryan]

On Tue, Jan 2, 2024 at 1:00 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Fri, Dec 29, 2023 at 06:56:07PM -0800, Suren Baghdasaryan wrote:
> > @@ -1078,9 +1078,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >
> >               /* at this point we have src_folio locked */
> >               if (folio_test_large(src_folio)) {
> > +                     /* split_folio() can block */
> > +                     pte_unmap(&orig_src_pte);
> > +                     pte_unmap(&orig_dst_pte);
> > +                     src_pte = dst_pte = NULL;
> >                       err = split_folio(src_folio);
> >                       if (err)
> >                               goto out;
> > +                     goto retry;
> >               }
>
> Do we also need to clear src_folio and src_folio_pte?  If the folio is a
> thp, I think it means it's pte mapped here. Then after the split we may
> want to fetch the small folio after the split, not the head one?

I think we need to re-fetch the src_folio only if the src_addr falls
into a non-head page. Looking at the __split_huge_page(), the head
page is skipped in the last loop, so I think it should stay valid.
That said, maybe it's just an implementation detail of the
__split_huge_page() and I should not rely on that and refetch anyway?

>
> --
> Peter Xu
>