Re: [PATCH] powerpc/64s/radix: Fix soft dirty tracking

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

Michael Ellerman <mpe@ellerman.id.au> writes:

> It was reported that soft dirty tracking doesn't work when using the
> Radix MMU.
>
> The tracking is supposed to work by clearing the soft dirty bit for a
> mapping and then write protecting the PTE. If/when the page is written
> to, a page fault occurs and the soft dirty bit is added back via
> pte_mkdirty(). For example in wp_page_reuse():
>
> 	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 	if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
> 		update_mmu_cache(vma, vmf->address, vmf->pte);
>
> Unfortunately on radix _PAGE_SOFTDIRTY is being dropped by
> radix__ptep_set_access_flags(), called from ptep_set_access_flags(),
> meaning the soft dirty bit is not set even though the page has been
> written to.
>
> Fix it by adding _PAGE_SOFTDIRTY to the set of bits that are able to be
> changed in radix__ptep_set_access_flags().
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

> Fixes: b0b5e9b13047 ("powerpc/mm/radix: Add radix pte #defines")
> Cc: stable@vger.kernel.org # v4.7+
> Reported-by: Dan Horák <dan@danny.cz>
> Link: https://lore.kernel.org/r/20230511095558.56663a50f86bdc4cd97700b7@danny.cz
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/mm/book3s64/radix_pgtable.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> index 26245aaf12b8..2297aa764ecd 100644
> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> @@ -1040,8 +1040,8 @@ void radix__ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
>  				  pte_t entry, unsigned long address, int psize)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
> -	unsigned long set = pte_val(entry) & (_PAGE_DIRTY | _PAGE_ACCESSED |
> -					      _PAGE_RW | _PAGE_EXEC);
> +	unsigned long set = pte_val(entry) & (_PAGE_DIRTY | _PAGE_SOFT_DIRTY |
> +					      _PAGE_ACCESSED | _PAGE_RW | _PAGE_EXEC);
>  
>  	unsigned long change = pte_val(entry) ^ pte_val(*ptep);
>  	/*
> -- 
> 2.40.1