linux-kernel - Re: [PATCH v5 8/9] mm/mmu_notifier: mmu_notifier_range_update_to_read

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Date:   Fri, 22 Feb 2019 14:42:08 -0800
From:   Ralph Campbell <rcampbell@...dia.com>
To:     <jglisse@...hat.com>, <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>,
        Christian König <christian.koenig@....com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>, Jan Kara <jack@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ross Zwisler <zwisler@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        John Hubbard <jhubbard@...dia.com>, <kvm@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <linux-rdma@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v5 8/9] mm/mmu_notifier:
 mmu_notifier_range_update_to_read_only() helper


On 2/19/19 12:04 PM, jglisse@...hat.com wrote:
> From: Jérôme Glisse <jglisse@...hat.com>
> 
> 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: Jérôme Glisse <jglisse@...hat.com>
> Cc: Christian König <christian.koenig@....com>
> Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
> Cc: Jani Nikula <jani.nikula@...ux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Felix Kuehling <Felix.Kuehling@....com>
> Cc: Jason Gunthorpe <jgg@...lanox.com>
> Cc: Ross Zwisler <zwisler@...nel.org>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Christian Koenig <christian.koenig@....com>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: kvm@...r.kernel.org
> Cc: dri-devel@...ts.freedesktop.org
> Cc: linux-rdma@...r.kernel.org
> Cc: Arnd Bergmann <arnd@...db.de>
> ---
>   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@...dia.com>