Re: [RFC 03/20] mm/mprotect: do not flush on permission promotion

[Nadav Amit <nadav.amit@gmail.com>]

> On Jan 30, 2021, at 5:07 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> Adding Andrew Cooper, who has a distressingly extensive understanding
> of the x86 PTE magic.
> 
> On Sat, Jan 30, 2021 at 4:16 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> Currently, using mprotect() to unprotect a memory region or uffd to
>> unprotect a memory region causes a TLB flush. At least on x86, as
>> protection is promoted, no TLB flush is needed.
>> 
>> Add an arch-specific pte_may_need_flush() which tells whether a TLB
>> flush is needed based on the old PTE and the new one. Implement an x86
>> pte_may_need_flush().
>> 
>> For x86, besides the simple logic that PTE protection promotion or
>> changes of software bits does require a flush, also add logic that
>> considers the dirty-bit. If the dirty-bit is clear and write-protect is
>> set, no TLB flush is needed, as x86 updates the dirty-bit atomically
>> on write, and if the bit is clear, the PTE is reread.
>> 
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Yu Zhao <yuzhao@google.com>
>> Cc: Nick Piggin <npiggin@gmail.com>
>> Cc: x86@kernel.org
>> ---
>> arch/x86/include/asm/tlbflush.h | 44 +++++++++++++++++++++++++++++++++
>> include/asm-generic/tlb.h       |  4 +++
>> mm/mprotect.c                   |  3 ++-
>> 3 files changed, 50 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
>> index 8c87a2e0b660..a617dc0a9b06 100644
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -255,6 +255,50 @@ static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
>> 
>> extern void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
>> 
>> +static inline bool pte_may_need_flush(pte_t oldpte, pte_t newpte)
>> +{
>> +       const pteval_t ignore_mask = _PAGE_SOFTW1 | _PAGE_SOFTW2 |
>> +                                    _PAGE_SOFTW3 | _PAGE_ACCESSED;
> 
> Why is accessed ignored?  Surely clearing the accessed bit needs a
> flush if the old PTE is present.

I am just following the current scheme in the kernel (x86):

int ptep_clear_flush_young(struct vm_area_struct *vma,
                           unsigned long address, pte_t *ptep)
{
        /*
         * On x86 CPUs, clearing the accessed bit without a TLB flush
         * doesn't cause data corruption. [ It could cause incorrect
         * page aging and the (mistaken) reclaim of hot pages, but the
         * chance of that should be relatively low. ]
         *
         * So as a performance optimization don't flush the TLB when
         * clearing the accessed bit, it will eventually be flushed by
         * a context switch or a VM operation anyway. [ In the rare
         * event of it not getting flushed for a long time the delay
         * shouldn't really matter because there's no real memory
         * pressure for swapout to react to. ]
         */
        return ptep_test_and_clear_young(vma, address, ptep);
}


> 
>> +       const pteval_t enable_mask = _PAGE_RW | _PAGE_DIRTY | _PAGE_GLOBAL;
>> +       pteval_t oldval = pte_val(oldpte);
>> +       pteval_t newval = pte_val(newpte);
>> +       pteval_t diff = oldval ^ newval;
>> +       pteval_t disable_mask = 0;
>> +
>> +       if (IS_ENABLED(CONFIG_X86_64) || IS_ENABLED(CONFIG_X86_PAE))
>> +               disable_mask = _PAGE_NX;
>> +
>> +       /* new is non-present: need only if old is present */
>> +       if (pte_none(newpte))
>> +               return !pte_none(oldpte);
>> +
>> +       /*
>> +        * If, excluding the ignored bits, only RW and dirty are cleared and the
>> +        * old PTE does not have the dirty-bit set, we can avoid a flush. This
>> +        * is possible since x86 architecture set the dirty bit atomically while
> 
> s/set/sets/
> 
>> +        * it caches the PTE in the TLB.
>> +        *
>> +        * The condition considers any change to RW and dirty as not requiring
>> +        * flush if the old PTE is not dirty or not writable for simplification
>> +        * of the code and to consider (unlikely) cases of changing dirty-bit of
>> +        * write-protected PTE.
>> +        */
>> +       if (!(diff & ~(_PAGE_RW | _PAGE_DIRTY | ignore_mask)) &&
>> +           (!(pte_dirty(oldpte) || !pte_write(oldpte))))
>> +               return false;
> 
> This logic seems confusing to me.  Is your goal to say that, if the
> old PTE was clean and writable and the new PTE is write-protected,
> then no flush is needed?

Yes.

> If so, I would believe you're right, but I'm
> not convinced you've actually implemented this.  Also, there may be
> other things going on that need flushing, e.g. a change of the address
> or an accessed bit or NX change.

The first part (diff & ~(_PAGE_RW | _PAGE_DIRTY | ignore_mask) is supposed
to capture changes of address, NX-bit, etc.

The second part is indeed wrong. It should have been:
 (!pte_dirty(oldpte) || !pte_write(oldpte))

> 
> Also, CET makes this extra bizarre.

I saw something about the not-writeable-and-dirty considered differently. I
need to have a look, but I am not sure it affects anything.