Re: [PATCH] mm: Fix modifying of page protection by insert_pfn()

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Jan Kara <jack@suse.cz> writes:

> Aneesh has reported that PPC triggers the following warning when
> excercising DAX code:
>
> [c00000000007610c] set_pte_at+0x3c/0x190
> LR [c000000000378628] insert_pfn+0x208/0x280
> Call Trace:
> [c0000002125df980] [8000000000000104] 0x8000000000000104 (unreliable)
> [c0000002125df9c0] [c000000000378488] insert_pfn+0x68/0x280
> [c0000002125dfa30] [c0000000004a5494] dax_iomap_pte_fault.isra.7+0x734/0xa40
> [c0000002125dfb50] [c000000000627250] __xfs_filemap_fault+0x280/0x2d0
> [c0000002125dfbb0] [c000000000373abc] do_wp_page+0x48c/0xa40
> [c0000002125dfc00] [c000000000379170] __handle_mm_fault+0x8d0/0x1fd0
> [c0000002125dfd00] [c00000000037a9b0] handle_mm_fault+0x140/0x250
> [c0000002125dfd40] [c000000000074bb0] __do_page_fault+0x300/0xd60
> [c0000002125dfe20] [c00000000000acf4] handle_page_fault+0x18
>
> Now that is WARN_ON in set_pte_at which is
>
>         VM_WARN_ON(pte_hw_valid(*ptep) && !pte_protnone(*ptep));
>
> The problem is that on some architectures set_pte_at() cannot cope with
> a situation where there is already some (different) valid entry present.
>
> Use ptep_set_access_flags() instead to modify the pfn which is built to
> deal with modifying existing PTE.
>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

> CC: stable@vger.kernel.org
> Fixes: b2770da64254 "mm: add vm_insert_mixed_mkwrite()"
> Reported-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/memory.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 47fe250307c7..ab650c21bccd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1549,10 +1549,12 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  				WARN_ON_ONCE(!is_zero_pfn(pte_pfn(*pte)));
>  				goto out_unlock;
>  			}
> -			entry = *pte;
> -			goto out_mkwrite;
> -		} else
> -			goto out_unlock;
> +			entry = pte_mkyoung(*pte);
> +			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +			if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> +				update_mmu_cache(vma, addr, pte);
> +		}
> +		goto out_unlock;
>  	}
>  
>  	/* Ok, finally just insert the thing.. */
> @@ -1561,7 +1563,6 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	else
>  		entry = pte_mkspecial(pfn_t_pte(pfn, prot));
>  
> -out_mkwrite:
>  	if (mkwrite) {
>  		entry = pte_mkyoung(entry);
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> -- 
> 2.16.4