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

Michael Ellerman mpe at ellerman.id.au
Thu May 11 21:42:24 AEST 2023

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().

Fixes: b0b5e9b13047 ("powerpc/mm/radix: Add radix pte #defines")
Cc: stable at vger.kernel.org # v4.7+
Reported-by: Dan Horák <dan at danny.cz>
Link: https://lore.kernel.org/r/20230511095558.56663a50f86bdc4cd97700b7@danny.cz
Signed-off-by: Michael Ellerman <mpe at 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);

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