Re: [PATCH v5 8/9] mm/mmu_notifier: mmu_notifier_range_update_to_read_only() helper

[Ralph Campbell]

On 2/19/19 12:04 PM, jglisse@xxxxxxxxxx wrote:
> From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
>
> Helper to test if a range is updated to read only (it is still valid
> to read from the range). This is useful for device driver or anyone
> who wish to optimize out update when they know that they already have
> the range map read only.
>
> Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> Cc: Christian KÃnig <christian.koenig@xxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Felix Kuehling <Felix.Kuehling@xxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: Ross Zwisler <zwisler@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Christian Koenig <christian.koenig@xxxxxxx>
> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-rdma@xxxxxxxxxxxxxxx
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>   include/linux/mmu_notifier.h |  4 ++++
>   mm/mmu_notifier.c            | 10 ++++++++++
>   2 files changed, 14 insertions(+)
>
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 0379956fff23..b6c004bd9f6a 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -259,6 +259,8 @@ extern void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *r,
>   				  bool only_end);
>   extern void __mmu_notifier_invalidate_range(struct mm_struct *mm,
>   				  unsigned long start, unsigned long end);
> +extern bool
> +mmu_notifier_range_update_to_read_only(const struct mmu_notifier_range *range);
>   
>   static inline bool
>   mmu_notifier_range_blockable(const struct mmu_notifier_range *range)
> @@ -568,6 +570,8 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm)
>   {
>   }
>   
> +#define mmu_notifier_range_update_to_read_only(r) false
> +
>   #define ptep_clear_flush_young_notify ptep_clear_flush_young
>   #define pmdp_clear_flush_young_notify pmdp_clear_flush_young
>   #define ptep_clear_young_notify ptep_test_and_clear_young
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index abd88c466eb2..ee36068077b6 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -395,3 +395,13 @@ void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
>   	mmdrop(mm);
>   }
>   EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);
> +
> +bool
> +mmu_notifier_range_update_to_read_only(const struct mmu_notifier_range *range)
> +{
> +	if (!range->vma || range->event != MMU_NOTIFY_PROTECTION_VMA)
> +		return false;
> +	/* Return true if the vma still have the read flag set. */
> +	return range->vma->vm_flags & VM_READ;
> +}
> +EXPORT_SYMBOL_GPL(mmu_notifier_range_update_to_read_only);

Don't you have to check for !WRITE & READ?
mprotect() can change the permissions from R/O to RW and
end up calling mmu_notifier_range_init() and 
mmu_notifier_invalidate_range_start()/end().

I'm not sure how useful this is since only applies to the
MMU_NOTIFY_PROTECTION_VMA case.
Anyway, you can add

Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>