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

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Message-ID: <20160809223004.GA7099@pc.thejh.net>
Date:	Wed, 10 Aug 2016 00:30:04 +0200
From:	Jann Horn <jann@...jh.net>
To:	Robert Foss <robert.foss@...labora.com>
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 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?

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