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

Andrew Morton
2023-06-09T20:06:38Z

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;

/*