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Subject: Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference
 count (semaphore)
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     mhiramat@kernel.org, peterz@infradead.org,
        srikar@linux.vnet.ibm.com, acme@kernel.org,
        ananth@linux.vnet.ibm.com, akpm@linux-foundation.org,
        alexander.shishkin@linux.intel.com, alexis.berlemont@gmail.com,
        corbet@lwn.net, dan.j.williams@intel.com,
        gregkh@linuxfoundation.org, huawei.libin@huawei.com,
        hughd@google.com, jack@suse.cz, jglisse@redhat.com,
        jolsa@redhat.com, kan.liang@intel.com,
        kirill.shutemov@linux.intel.com, kjlx@templeofstupid.com,
        kstewart@linuxfoundation.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com,
        milian.wolff@kdab.com, mingo@redhat.com, namhyung@kernel.org,
        naveen.n.rao@linux.vnet.ibm.com, pc@us.ibm.com,
        pombredanne@nexb.com, rostedt@goodmis.org, tglx@linutronix.de,
        tmricht@linux.vnet.ibm.com, willy@infradead.org,
        yao.jin@linux.intel.com, fengguang.wu@intel.com,
        Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
References: <20180313125603.19819-1-ravi.bangoria@linux.vnet.ibm.com>
 <20180313125603.19819-6-ravi.bangoria@linux.vnet.ibm.com>
 <20180315150156.GA19767@redhat.com>
From:   Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Date:   Fri, 16 Mar 2018 15:01:05 +0530
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On 03/15/2018 08:31 PM, Oleg Nesterov wrote:
> On 03/13, Ravi Bangoria wrote:
>> +sdt_update_ref_ctr(struct mm_struct *mm, unsigned long vaddr, short d)
>> +{
>> +	void *kaddr;
>> +	struct page *page;
>> +	struct vm_area_struct *vma;
>> +	int ret = 0;
>> +	unsigned short orig = 0;
>> +
>> +	if (vaddr == 0)
>> +		return -EINVAL;
>> +
>> +	ret = get_user_pages_remote(NULL, mm, vaddr, 1,
>> +		FOLL_FORCE | FOLL_WRITE, &page, &vma, NULL);
>> +	if (ret <= 0)
>> +		return ret;
>> +
>> +	kaddr = kmap_atomic(page);
>> +	memcpy(&orig, kaddr + (vaddr & ~PAGE_MASK), sizeof(orig));
>> +	orig += d;
>> +	memcpy(kaddr + (vaddr & ~PAGE_MASK), &orig, sizeof(orig));
>> +	kunmap_atomic(kaddr);
> Hmm. Why memcpy? You could simply do
>
> 	kaddr = kmap_atomic();
> 	unsigned short *ptr = kaddr + (vaddr & ~PAGE_MASK);
> 	*ptr += d;
> 	kunmap_atomic();

Yes, that should work. Will change it.

Thanks for the review,
Ravi