Re: [RFC PATCH 03/14] mm/mprotect: allow exclusive anon pages to be writable

David Hildenbrand
2022-07-20T15:20:10Z

On 18.07.22 14:02, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
>
> Anonymous pages might have the dirty bit clear, but this should not
> prevent mprotect from making them writable if they are exclusive.
> Therefore, skip the test whether the page is dirty in this case.
>
> 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>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> mm/mprotect.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 34c2dfb68c42..da5b9bf8204f 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -45,7 +45,7 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
>
> VM_BUG_ON(!(vma->vm_flags & VM_WRITE) || pte_write(pte));
>
> - if (pte_protnone(pte) || !pte_dirty(pte))
> + if (pte_protnone(pte))
> return false;
>
> /* Do we need write faults for softdirty tracking? */
> @@ -66,7 +66,8 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
> page = vm_normal_page(vma, addr, pte);
> if (!page || !PageAnon(page) || !PageAnonExclusive(page))
> return false;
> - }
> + } else if (!pte_dirty(pte))
> + return false;
>
> return true;
> }

When I wrote that code, I was wondering how often that would actually
happen in practice -- and if we care about optimizing that. Do you have
a gut feeling in which scenarios this would happen and if we care?

If the page is in the swapcache and was swapped out, you'd be requiring
a writeback even though nobody modified the page and possibly isn't
going to do so in the near future.

--
Thanks,

David / dhildenb