linux-kernel - Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps

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Date:	Wed, 10 Aug 2016 10:16:45 -0400
From:	Robert Foss <robert.foss@...labora.com>
To:	Jann Horn <jann@...jh.net>
Cc:	Sonny Rao <sonnyrao@...omium.org>, akpm@...ux-foundation.org,
	keescook@...omium.org, viro@...iv.linux.org.uk,
	gorcunov@...nvz.org, john.stultz@...aro.org,
	plaguedbypenguins@...il.com, mguzik@...hat.com,
	adobriyan@...il.com, jdanis@...gle.com, calvinowens@...com,
	mhocko@...e.com, koct9i@...il.com, vbabka@...e.cz,
	n-horiguchi@...jp.nec.com, kirill.shutemov@...ux.intel.com,
	ldufour@...ux.vnet.ibm.com, hannes@...xchg.org,
	linux-kernel@...r.kernel.org, Ben Zhang <benzh@...omium.org>,
	Bryan Freed <bfreed@...omium.org>,
	Filipe Brandenburger <filbranden@...omium.org>
Subject: Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps



On 2016-08-09 06:30 PM, Jann Horn wrote:
> On Tue, Aug 09, 2016 at 05:01:44PM -0400, Robert Foss wrote:
>> On 2016-08-09 03:24 PM, Jann Horn wrote:
>>> On Tue, Aug 09, 2016 at 12:05:43PM -0400, robert.foss@...labora.com wrote:
>>>> +	down_read(&mm->mmap_sem);
>>>> +	hold_task_mempolicy(priv);
>>>> +
>>>> +	for (vma = mm->mmap; vma != priv->tail_vma; vma = vma->vm_next) {
>>>> +		struct mem_size_stats mss;
>>>> +		struct mm_walk smaps_walk = {
>>>> +			.pmd_entry = smaps_pte_range,
>>>> +			.mm = vma->vm_mm,
>>>> +			.private = &mss,
>>>> +		};
>>>> +
>>>> +		if (vma->vm_mm && !is_vm_hugetlb_page(vma)) {
>>>> +			memset(&mss, 0, sizeof(mss));
>>>> +			walk_page_vma(vma, &smaps_walk);
>>>> +			add_smaps_sum(&mss, mss_sum);
>>>> +		}
>>>> +	}
>>>
>>> Errrr... what? You accumulate values from mem_size_stats items into a
>>> struct mss_sum that is associated with the struct file? So when you
>>> read the file the second time, you get the old values plus the new ones?
>>> And when you read the file in parallel, you get inconsistent values?
>>>
>>> For most files in procfs, the behavior is that you can just call
>>> pread(fd, buf, sizeof(buf), 0) on the same fd again and again, giving
>>> you the current values every time, without mutating state. I strongly
>>> recommend that you get rid of priv->mss and just accumulate the state
>>> in a local variable (maybe one on the stack).
>>
>> So a simple "static struct mem_size_stats" in totmaps_proc_show() would be a
>> better solution?
>
> Er, why "static"? Are you trying to create shared state between different
> readers for some reason?
>

I think I'm a bit confused now, how are you suggesting that I replace 
priv->mss?