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Subject: Re: [PATCH v3 1/3] mm/mprotect: Fix soft-dirty check in
 can_change_pte_writable()
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <e35e42ce-e942-141d-09e7-a3a7868f4abb@redhat.com>
Date:   Fri, 22 Jul 2022 10:21:00 -0700
Cc:     Peter Xu <peterx@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>
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References: <20220721183338.27871-1-peterx@redhat.com>
 <20220721183338.27871-2-peterx@redhat.com>
 <e35e42ce-e942-141d-09e7-a3a7868f4abb@redhat.com>
To:     David Hildenbrand <david@redhat.com>
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On Jul 22, 2022, at 12:08 AM, David Hildenbrand <david@redhat.com> wrote:

>> +static inline bool vma_soft_dirty_enabled(struct vm_area_struct *vma)
>> +{
>> +	/*
>> +	 * NOTE: we must check this before VM_SOFTDIRTY on soft-dirty
>> +	 * enablements, because when without soft-dirty being compiled in,
>> +	 * VM_SOFTDIRTY is defined as 0x0, then !(vm_flags & VM_SOFTDIRTY)
>> +	 * will be constantly true.
>> +	 */
>> +	if (!IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
>> +		return false;
>> +
>> +	/*
>> +	 * Soft-dirty is kind of special: its tracking is enabled when the
>> +	 * vma flags not set.
>> +	 */
>> +	return !(vma->vm_flags & VM_SOFTDIRTY);
>> +}
> 
> That will come in handy in other patches I'm cooking.

clear_refs_write() also comes to mind as well (for consistency; I see no
correctness or performance issue).