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Date: Mon, 5 Oct 2015 09:53:18 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, Jerome Marchand <jmarchan@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, Michal Hocko <mhocko@suse.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Randy Dunlap <rdunlap@infradead.org>, linux-s390@vger.kernel.org,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, Linux API <linux-api@vger.kernel.org>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: [PATCH v4 2/4] mm, proc: account for shmem swap in
 /proc/pid/smaps
Message-ID: <20151005075318.GE2903@worktop.programming.kicks-ass.net>
References: <1443792951-13944-1-git-send-email-vbabka@suse.cz>
 <1443792951-13944-3-git-send-email-vbabka@suse.cz>
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On Fri, Oct 02, 2015 at 03:35:49PM +0200, Vlastimil Babka wrote:
> +static unsigned long smaps_shmem_swap(struct vm_area_struct *vma)
> +{
> +	struct inode *inode;
> +	unsigned long swapped;
> +	pgoff_t start, end;
> +
> +	if (!vma->vm_file)
> +		return 0;
> +
> +	inode = file_inode(vma->vm_file);
> +
> +	if (!shmem_mapping(inode->i_mapping))
> +		return 0;
> +
> +	/*
> +	 * The easier cases are when the shmem object has nothing in swap, or
> +	 * we have the whole object mapped. Then we can simply use the stats
> +	 * that are already tracked by shmem.
> +	 */
> +	swapped = shmem_swap_usage(inode);
> +
> +	if (swapped == 0)
> +		return 0;
> +
> +	if (vma->vm_end - vma->vm_start >= inode->i_size)
> +		return swapped;
> +
> +	/*
> +	 * Here we have to inspect individual pages in our mapped range to
> +	 * determine how much of them are swapped out. Thanks to RCU, we don't
> +	 * need i_mutex to protect against truncating or hole punching.
> +	 */

At the very least put in an assertion that we hold the RCU read lock,
otherwise RCU doesn't guarantee anything and its not obvious it is held
here.

> +	start = linear_page_index(vma, vma->vm_start);
> +	end = linear_page_index(vma, vma->vm_end);
> +
> +	return shmem_partial_swap_usage(inode->i_mapping, start, end);
> +}

> + * Determine (in bytes) how much of the whole shmem object is swapped out.
> + */
> +unsigned long shmem_swap_usage(struct inode *inode)
> +{
> +	struct shmem_inode_info *info = SHMEM_I(inode);
> +	unsigned long swapped;
> +
> +	/* Mostly an overkill, but it's not atomic64_t */

Yeah, that don't make any kind of sense.

> +	spin_lock(&info->lock);
> +	swapped = info->swapped;
> +	spin_unlock(&info->lock);
> +
> +	return swapped << PAGE_SHIFT;
> +}