drivers/char/mem.c: avoid OOM lockup during large reads from /dev/zero

commit 730c586ad5228c339949b2eb4e72b80ae167abc4 upstream.

While running 20 parallel instances of dd as follows:

  #!/bin/bash
  for i in `seq 1 20`; do
           dd if=/dev/zero of=/export/hda3/dd_$i bs=1073741824 count=1 &
  done
  wait

on a 16G machine, we noticed that rather than just killing the processes,
the entire kernel went down.  Stracing dd reveals that it first does an
mmap2, which makes 1GB worth of zero page mappings.  Then it performs a
read on those pages from /dev/zero, and finally it performs a write.

The machine died during the reads.  Looking at the code, it was noticed
that /dev/zero's read operation had been changed by
557ed1fa2620dc119adb86b34c614e152a629a80 ("remove ZERO_PAGE") from giving
zero page mappings to actually zeroing the page.

The zeroing of the pages causes physical pages to be allocated to the
process.  But, when the process exhausts all the memory that it can, the
kernel cannot kill it, as it is still in the kernel mode allocating more
memory.  Consequently, the kernel eventually crashes.

To fix this, I propose that when a fatal signal is pending during
/dev/zero read operation, we simply return and let the user process die.

Signed-off-by: Salman Qazi <sqazi@google.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Modified error return and comment trivially.  - Linus]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 672b08e694d05cd041e8a3bbac17264f3ca2fb4e..3191fc8440abf2c0b4fc494b61728852c8ec6bec 100644 (file)
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -724,6 +724,9 @@ static ssize_t read_zero(struct file * file, char __user * buf,
                written += chunk - unwritten;
                if (unwritten)
                        break;
+               /* Consider changing this to just 'signal_pending()' with lots of testing */
+               if (fatal_signal_pending(current))
+                       return written ? written : -EINTR;
                buf += chunk;
                count -= chunk;
                cond_resched();