linux-kernel - When booting a 16TB system, unix_create1 fails due to integer overflow.

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Date:	Thu, 23 Sep 2010 07:17:04 -0500
From:	Robin Holt <holt@....com>
To:	Al Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>,
	"Denis V. Lunev" <den@...nvz.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Miklos Szeredi <mszeredi@...e.cz>,
	Mingming Cao <cmm@...ibm.com>, Nick Piggin <npiggin@...nel.dk>,
	Pavel Emelyanov <xemul@...nvz.org>
Cc:	holt@....com, linux-kernel@...r.kernel.org
Subject: When booting a 16TB system, unix_create1 fails due to integer
 overflow.

I do not know which direction to take, but here is the summary of the
problem.

We recently started trying to boot a customer's two new machines which
are configured with 384GB short of 16TB of memory.

We were seeing a failure which prevented boot.  The kernel was incapable
of creating either a named pipe or unix domain socket.  This comes down
to a common kernel function called unix_create1() which does:

	atomic_inc(&unix_nr_socks);
	if (atomic_read(&unix_nr_socks) > 2 * get_max_files())
		goto out;

The function get_max_files() is a simple return of files_stat.max_files.
files_stat.max_files is a signed integer and is computed in
fs/file_table.c's files_init().

	n = (mempages * (PAGE_SIZE / 1024)) / 10;
	files_stat.max_files = n;

In our case, mempages (total_ram_pages) is approx 3,758,096,384
(0xe0000000).  That leaves max_files at approximately 1,503,238,553.
This causes 2 * get_max_files() to integer overflow.

We came up with a few possible solutions:

Our first response was to limit max_files to (INT_MAX / 2)  This at
least got us past the problem and seemed reasonable.

We could also have changed the 2 * get_max_files() to 2UL *
get_max_files() and gotten past this point in boot.  That was not tested.

We could also have changed the definition of max_files to at least an
unsigned int instead of an int and gotten past the problem, but again,
not tested.

Any suggestions for a direction would be appreciated.

Thank you,
Robin Holt
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