From ea410d9109f242afb1e1de2d33802c4900bd0b99 Mon Sep 17 00:00:00 2001 From: Salman Qazi Date: Thu, 4 Jun 2009 15:20:39 -0700 Subject: [PATCH] drivers/char/mem.c: avoid OOM lockup during large reads from /dev/zero 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 f9216d29ec734f1f1a0a008fbbd8cc2cb256211b ("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 Cc: Nick Piggin Signed-off-by: Andrew Morton [ Modified error return and comment trivially. - Linus] Signed-off-by: Linus Torvalds --- drivers/char/mem.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 8f05c38c2f063..65e12bca657cf 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -694,6 +694,9 @@ 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(); -- 2.39.5