Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap_free()

Catalin Marinas
2023-05-15T17:37:41Z

On Sat, May 13, 2023 at 05:29:53AM +0200, David Hildenbrand wrote:
> On 13.05.23 01:57, Peter Collingbourne wrote:
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 01a23ad48a04..83268d287ff1 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3914,19 +3914,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > }
> > }
> > - /*
> > - * Remove the swap entry and conditionally try to free up the swapcache.
> > - * We're already holding a reference on the page but haven't mapped it
> > - * yet.
> > - */
> > - swap_free(entry);
> > - if (should_try_to_free_swap(folio, vma, vmf->flags))
> > - folio_free_swap(folio);
> > -
> > - inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > - dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> > pte = mk_pte(page, vma->vm_page_prot);
> > -
> > /*
> > * Same logic as in do_wp_page(); however, optimize for pages that are
> > * certainly not shared either because we just allocated them without
> > @@ -3946,8 +3934,21 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > pte = pte_mksoft_dirty(pte);
> > if (pte_swp_uffd_wp(vmf->orig_pte))
> > pte = pte_mkuffd_wp(pte);
> > + arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > vmf->orig_pte = pte;
> > + /*
> > + * Remove the swap entry and conditionally try to free up the swapcache.
> > + * We're already holding a reference on the page but haven't mapped it
> > + * yet.
> > + */
> > + swap_free(entry);
> > + if (should_try_to_free_swap(folio, vma, vmf->flags))
> > + folio_free_swap(folio);
> > +
> > + inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> > + dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> > +
> > /* ksm created a completely new copy */
> > if (unlikely(folio != swapcache && swapcache)) {
> > page_add_new_anon_rmap(page, vma, vmf->address);
> > @@ -3959,7 +3960,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > VM_BUG_ON(!folio_test_anon(folio) ||
> > (pte_write(pte) && !PageAnonExclusive(page)));
> > set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> > - arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > folio_unlock(folio);
> > if (folio != swapcache && swapcache) {
>
>
> You are moving the folio_free_swap() call after the folio_ref_count(folio)
> == 1 check, which means that such (previously) swapped pages that are
> exclusive cannot be detected as exclusive.
>
> There must be a better way to handle MTE here.
>
> Where are the tags stored, how is the location identified, and when are they
> effectively restored right now?

I haven't gone through Peter's patches yet but a pretty good description
of the problem is here:
https://lore.kernel.org/all/5050805753ac469e8d727c797c2218a9d780d434.camel@mediatek.com/.
I couldn't reproduce it with my swap setup but both Qun-wei and Peter
triggered it.

When a tagged page is swapped out, the arm64 code stores the metadata
(tags) in a local xarray indexed by the swap pte. When restoring from
swap, the arm64 set_pte_at() checks this xarray using the old swap pte
and spills the tags onto the new page. Apparently something changed in
the kernel recently that causes swap_range_free() to be called before
set_pte_at(). The arm64 arch_swap_invalidate_page() frees the metadata
from the xarray and the subsequent set_pte_at() won't find it.

If we have the page, the metadata can be restored before set_pte_at()
and I guess that's what Peter is trying to do (again, I haven't looked
at the details yet; leaving it for tomorrow).

Is there any other way of handling this? E.g. not release the metadata
in arch_swap_invalidate_page() but later in set_pte_at() once it was
restored. But then we may leak this metadata if there's no set_pte_at()
(the process mapping the swap entry died).

--
Catalin