On 11/27/20 3:14 PM, Catalin Marinas wrote: > 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)))); This seems like a generic enough principle which all platforms should adhere to. But the proposed test WARN_ON(pte_dirty(pte_wrprotect(pte))) might fail on some platforms if the page table entry came in as a dirty one and pte_wrprotect() is not expected to alter the dirty state. Instead, should we just add the following two tests, which would ensure that pte_wrprotect() never alters the dirty state of a page table entry. WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte)))); WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(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)))); Right.
