Re: [PATCH v2 28/32] mm/memory: allow pte_offset_map[_lock]() to fail

Hugh Dickins
2023-06-09T20:11:41Z

On Fri, 9 Jun 2023, Andrew Morton wrote:
> On Thu, 8 Jun 2023 18:43:38 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
>
> > copy_pte_range(): use pte_offset_map_nolock(), and allow for it to fail;
> > but with a comment on some further assumptions that are being made there.
> >
> > zap_pte_range() and zap_pmd_range(): adjust their interaction so that
> > a pte_offset_map_lock() failure in zap_pte_range() leads to a retry in
> > zap_pmd_range(); remove call to pmd_none_or_trans_huge_or_clear_bad().
> >
> > Allow pte_offset_map_lock() to fail in many functions. Update comment
> > on calling pte_alloc() in do_anonymous_page(). Remove redundant calls
> > to pmd_trans_unstable(), pmd_devmap_trans_unstable(), pmd_none() and
> > pmd_bad(); but leave pmd_none_or_clear_bad() calls in free_pmd_range()
> > and copy_pmd_range(), those do simplify the next level down.
> >
> > ...
> >
> > @@ -3728,11 +3737,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > vmf->page = pfn_swap_entry_to_page(entry);
> > vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> > vmf->address, &vmf->ptl);
> > - if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
> > - spin_unlock(vmf->ptl);
> > - goto out;
> > - }
> > -
> > + if (unlikely(!vmf->pte ||
> > + !pte_same(*vmf->pte, vmf->orig_pte)))
> > + goto unlock;
> > /*
> > * Get a page reference while we know the page can't be
> > * freed.
>
> This hunk falls afoul of
> https://lkml.kernel.org/r/20230602092949.545577-5-ryan.roberts@arm.com.
>
> I did this:
>
> @@ -3729,7 +3738,8 @@ vm_fault_t do_swap_page(struct vm_fault
> vmf->page = pfn_swap_entry_to_page(entry);
> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> vmf->address, &vmf->ptl);
> - if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
> + if (unlikely(!vmf->pte ||
> + !pte_same(*vmf->pte, vmf->orig_pte)))
> goto unlock;
>
> /*

Yes, that's exactly right: thanks, Andrew.

Hugh