Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

[Nadav Amit <namit@vmware.com>]

On Jul 20, 2022, at 2:42 AM, David Hildenbrand <david@redhat.com> wrote:

> ⚠ External Email
> 
> On 18.07.22 14:01, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> When userfaultfd makes a PTE writable, it can now change the PTE
>> directly, in some cases, without going triggering a page-fault first.
>> Yet, doing so might leave the PTE that was write-unprotected as old and
>> clean. At least on x86, this would cause a >500 cycles overhead when the
>> PTE is first accessed.
>> 
>> Use MM_CP_WILL_NEED to set the PTE as young and dirty when userfaultfd
>> gets a hint that the page is likely to be used. Avoid changing the PTE
>> to young and dirty in other cases to avoid excessive writeback and
>> messing with the page reclamation logic.
>> 
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Peter Xu <peterx@redhat.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>
>> ---
>> include/linux/mm.h | 2 ++
>> mm/mprotect.c | 9 ++++++++-
>> mm/userfaultfd.c | 8 ++++++--
>> 3 files changed, 16 insertions(+), 3 deletions(-)
>> 
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 9cc02a7e503b..4afd75ce5875 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1988,6 +1988,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
>> /* Whether this change is for write protecting */
>> #define MM_CP_UFFD_WP (1UL << 2) /* do wp */
>> #define MM_CP_UFFD_WP_RESOLVE (1UL << 3) /* Resolve wp */
>> +/* Whether to try to mark entries as dirty as they are to be written */
>> +#define MM_CP_WILL_NEED (1UL << 4)
>> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
>> MM_CP_UFFD_WP_RESOLVE)
>> 
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 996a97e213ad..34c2dfb68c42 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -82,6 +82,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>> bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
>> bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
>> bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
>> + bool will_need = cp_flags & MM_CP_WILL_NEED;
>> 
>> tlb_change_page_size(tlb, PAGE_SIZE);
>> 
>> @@ -172,6 +173,9 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>> ptent = pte_clear_uffd_wp(ptent);
>> }
>> 
>> + if (will_need)
>> + ptent = pte_mkyoung(ptent);
>> +
>> /*
>> * In some writable, shared mappings, we might want
>> * to catch actual write access -- see
>> @@ -187,8 +191,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>> */
>> if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>> !pte_write(ptent) &&
> 
> 
> Why would we want to check if we can set something writable if it
> already *is* writable? That doesn't make sense to me.

We check !pte_write(). What am I missing in your question?

Having said that, I do notice now that pte_mkdirty() should not be done
only this condition is fulfilled. Instead we should just have
something like:

                       if (will_need) {
                               ptent = pte_mkyoung(ptent);
                               if (pte_write(ptent))
                                       ptent = pte_mkdirty(ptent);
                       }

But I do not think this answers your question, which I did not understand.