On Fri, Nov 27, 2020 at 09:22:24AM +0100, Christophe Leroy wrote: > Le 27/11/2020 à 06:06, Anshuman Khandual a écrit : > > This adds validation tests for dirtiness after write protect conversion for > > each page table level. This is important for platforms such as arm64 that > > removes the hardware dirty bit while making it an write protected one. This > > also fixes pxx_wrprotect() related typos in the documentation file. > > > diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c > > index c05d9dcf7891..a5be11210597 100644 > > --- a/mm/debug_vm_pgtable.c > > +++ b/mm/debug_vm_pgtable.c > > @@ -70,6 +70,7 @@ static void __init pte_basic_tests(unsigned long pfn, pgprot_t prot) > > WARN_ON(pte_young(pte_mkold(pte_mkyoung(pte)))); > > WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte)))); > > WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte)))); > > + WARN_ON(pte_dirty(pte_wrprotect(pte))); > > Wondering what you are testing here exactly. > > Do you expect that if PTE has the dirty bit, it gets cleared by pte_wrprotect() ? > > Powerpc doesn't do that, it only clears the RW bit but the dirty bit remains > if it is set, until you call pte_mkclean() explicitely. Arm64 has an unusual way of setting a hardware dirty "bit", it actually clears the PTE_RDONLY bit. The pte_wrprotect() sets the PTE_RDONLY bit back and we can lose the dirty information. Will found this and posted patches to fix the arm64 pte_wprotect() to set a software PTE_DIRTY if !PTE_RDONLY (we do this for ptep_set_wrprotect() already). My concern was that we may inadvertently make a fresh/clean pte dirty with such change, hence the suggestion for the test. That said, I think we also need a test in the other direction, pte_wrprotect() should preserve any dirty information: WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); If pte_mkwrite() makes a pte truly writable and potentially dirty, we could also add a test as below. However, I think that's valid for arm64, other architectures with a separate hardware dirty bit would fail this: WARN_ON(!pte_dirty(pte_wrprotect(pte_mkwrite(pte)))); -- Catalin
