Re: [PATCH -V6 00/21] swap: Swapout/swapin THP in one piece

Daniel Jordan
2018-10-23T12:27:59Z

On Wed, Oct 10, 2018 at 03:19:03PM +0800, Huang Ying wrote:
> And for all, Any comment is welcome!
>
> This patchset is based on the 2018-10-3 head of mmotm/master.

There seems to be some infrequent memory corruption with THPs that have been
swapped out: page contents differ after swapin.

Reproducer at the bottom. Part of some tests I'm writing, had to separate it a
little hack-ily. Basically it writes the word offset _at_ each word offset in
a memory blob, tries to push it to swap, and verifies the offset is the same
after swapin.

I ran with THP enabled=always. THP swapin_enabled could be always or never, it
happened with both. Every time swapping occurred, a single THP-sized chunk in
the middle of the blob had different offsets. Example:

** > word corruption gap
** corruption detected 14929920 bytes in (got 15179776, expected 14929920) **
** corruption detected 14929928 bytes in (got 15179784, expected 14929928) **
** corruption detected 14929936 bytes in (got 15179792, expected 14929936) **
...pattern continues...
** corruption detected 17027048 bytes in (got 15179752, expected 17027048) **
** corruption detected 17027056 bytes in (got 15179760, expected 17027056) **
** corruption detected 17027064 bytes in (got 15179768, expected 17027064) **
100.0% of memory was swapped out at mincore time
0.00305% of pages were corrupted (first corrupt word 14929920, last corrupt word 17027064)

The problem goes away with THP enabled=never, and I don't see it on 2018-10-3
mmotm/master with THP enabled=always.

The server had an NVMe swap device and ~760G memory over two nodes, and the
program was always run like this: swap-verify -s $((64 * 2**30))

The kernels had one extra patch, Alexander Duyck's
"dma-direct: Fix return value of dma_direct_supported", which was required to
get them to build.

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/*
* swap-verify.c - helper to verify contents of swapped out pages
*
* Daniel Jordan <daniel.m.jordan@oracle.com>
*/

#define _GNU_SOURCE
#include <getopt.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>

#define TEST_SUCCESS 0
#define TEST_FAIL 1
#define TEST_SKIP 2

static void usagedie(int exitcode)
{
fprintf(stderr, "usage: swap-verify\n"
" -h show this message\n"
" -s bytes\n");
exit(exitcode);
}

int main(int argc, char **argv)
{
int c, pgsize;
char *pages;
unsigned char *mincore_vec;
size_t i, j, nr_pages_swapped, nr_pages_corrupted;
size_t bytes = 1ul << 30; /* default 1G */
ssize_t bytes_read;
size_t first_corrupt_word, last_corrupt_word, prev_corrupt_word;

while ((c = getopt(argc, argv, "hs:")) != -1) {
switch (c) {
case 'h':
usagedie(0);
break;
case 's':
bytes = strtoul(optarg, NULL, 10);
break;
default:
fprintf(stderr, "unrecognized option %c\n", c);
exit(TEST_SKIP);
}
}

pgsize = getpagesize();

if ((mincore_vec = calloc(bytes / pgsize, 1)) == NULL) {
perror("calloc");
exit(TEST_SKIP);
}

if ((pages = mmap(NULL, bytes, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)) == MAP_FAILED) {
perror("mmap");
exit(TEST_SKIP);
}

/* Fill pages with a "random" pattern. */
for (i = 0; i < bytes; i += sizeof(unsigned long))
*(unsigned long *)(pages + i) = i;

/* Now fill memory, trying to push the pages just allocated to swap. */
system("./use-mem-total");

/* Is the memory swapped out? */
if (mincore(pages, bytes, mincore_vec) == -1) {
perror("mincore");
exit(TEST_SKIP);
}

nr_pages_swapped = 0;
nr_pages_corrupted = 0;
first_corrupt_word = bytes;
last_corrupt_word = 0;
prev_corrupt_word = bytes;
for (i = 0; i < bytes; i += pgsize) {
bool page_corrupt = false;
if (mincore_vec[i / pgsize] & 1) {
/* Resident, don't bother checking. */
continue;
}
++nr_pages_swapped;
for (j = i; j < i + pgsize; j += sizeof(unsigned long)) {
unsigned long val = *(unsigned long *)(pages + j);
if (val != j) {
if (!page_corrupt)
++nr_pages_corrupted;
page_corrupt = true;
if (j - prev_corrupt_word != sizeof(unsigned long))
fprintf(stderr, "** > word corruption gap\n");
if (j % (1ul << 21) == 0)
fprintf(stderr, "-- THP boundary\n");
if (j < first_corrupt_word)
first_corrupt_word = j;
if (j > last_corrupt_word)
last_corrupt_word = j;
fprintf(stderr, "** corruption detected %lu "
"bytes in (got %lu, expected %lu) **\n",
j, val, j);
prev_corrupt_word = j;
}
}
}
fprintf(stderr, "%.1f%% of memory was swapped out at mincore time\n",
((double)nr_pages_swapped / (bytes / pgsize)) * 100);

if (nr_pages_corrupted) {
fprintf(stderr, "%.5f%% of pages were corrupted (first corrupt "
"word %lu, last corrupt word %lu)\n",
((double)nr_pages_corrupted / (bytes / pgsize)) * 100,
first_corrupt_word, last_corrupt_word);
} else {
fprintf(stderr, "no memory corruption detected\n");
}
return (nr_pages_corrupted) ? TEST_FAIL : TEST_SUCCESS;
}

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#!/usr/bin/env bash
#
# use-mem-total
#
# Helper that allocates MemTotal and exits immediately. Useful for causing
# swapping of previously allocated memory.

# XXX fix paths
source /path/to/vm-scalability/hw_vars

/path/to/usemem --thread $nr_task --step $pagesize -q --repeat 4 \
$(( mem * 11 / 10 / nr_task )) > /dev/null

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