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Date:   Fri, 9 Jun 2023 13:06:32 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Hugh Dickins <hughd@google.com>
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        Mike Rapoport <rppt@kernel.org>,
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        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2 28/32] mm/memory: allow pte_offset_map[_lock]() to
 fail
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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;
 
 			/*