linux-kernel - Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference count (semaphore)

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Message-ID: <20180315150156.GA19767@redhat.com>
Date:   Thu, 15 Mar 2018 16:01:57 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Cc:     mhiramat@...nel.org, peterz@...radead.org,
        srikar@...ux.vnet.ibm.com, acme@...nel.org,
        ananth@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        alexander.shishkin@...ux.intel.com, alexis.berlemont@...il.com,
        corbet@....net, dan.j.williams@...el.com,
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        kirill.shutemov@...ux.intel.com, kjlx@...pleofstupid.com,
        kstewart@...uxfoundation.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
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        yao.jin@...ux.intel.com, fengguang.wu@...el.com
Subject: Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference
 count (semaphore)

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();

Oleg.