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

From: Catalin Marinas <[email-masked]>
Date: Mon, 15 May 2023 18:34:30 +0100

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/[email-masked]/.
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

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