ask for your help about a patch (commit: 9845cbb)

Hi Kirill,

Currently I'm doing some kernel test work, including that reproducing some existing kernel bugs. Here I may need your some help.

On 2014-02-25, you committed a patch (commit: 9845cbb) about thp.

mm, thp: fix infinite loop on memcg OOM
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Masayoshi Mizuma reported a bug with the hang of an application under the memcg limit.
It happens on write-protection fault to huge zero page.

If we successfully allocate a huge page to replace zero page but hit the memcg limit we
need to split the zero page with split_huge_page_pmd() and fallback to small pages.

The other part of the problem is that VM_FAULT_OOM has special meaning in do_huge_pmd_wp_page() context. __handle_mm_fault() expects the page to be split if
it sees VM_FAULT_OOM and it will will retry page fault handling. This causes an infinite loop
if the page was not split.

do_huge_pmd_wp_zero_page_fallback() can return VM_FAULT_OOM if it failed to allocate
one small page, so fallback to small pages will not help.

The solution for this part is to replace VM_FAULT_OOM with VM_FAULT_FALLBACK is
fallback required.
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It is a little difficult to reproduce this problem fixed by this patch for me. Could you give me some
hint about how to do this - a??allocate a huge page to replace zero page but hit the memcg limit"?

Quoted 1 line · gux.fnst

Hi Kirill,

Hi Xing,

Please always CC to mailing list for upstream-related questions. I've added linux-mm@ to CC.

VM_FAULT_FALLBACK is
fallback required.
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It is a little difficult to reproduce this problem fixed by this patch for me. Could you give me some
hint about how to do this - “allocate a huge page to replace zero page but hit the memcg limit"?
I used this script:

#!/bin/sh -efu

set -efux

mount -t cgroup none /sys/fs/cgroup
mkdir /sys/fs/cgroup/test
echo "10M" > /sys/fs/cgroup/test/memory.limit_in_bytes echo "10M" > /sys/fs/cgroup/test/memory.memsw.limit_in_bytes

echo $$ > /sys/fs/cgroup/test/tasks
/host/home/kas/var/mmaptest_zero
echo ok

Where /host/home/kas/var/mmaptest_zero is:

#include <assert.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>

#define MB (1024 * 1024)
#define SIZE (256 * MB)

int main(int argc, char **argv)

{
	int i;
	char *p;

	posix_memalign((void **)&p, 2 * MB, SIZE);
	printf("p: %p\n", p);
	fork();
	for (i = 0; i < SIZE; i += 4096)
		assert(p[i] == 0);

	for (i = 0; i < SIZE; i += 4096)
		p[i] = 1;

	pause();
	return 0;
}

Without the patch it hangs, but should trigger OOM.