On 11/24/22 6:43 PM, David Hildenbrand wrote:
On 24.11.22 11:21, Gavin Shan wrote:
On 11/24/22 6:09 PM, David Hildenbrand wrote:
On 24.11.22 10:55, Gavin Shan wrote:
The issue is reported when removing memory through virtio_mem device.
The transparent huge page, experienced copy-on-write fault, is wrongly
regarded as pinned. The transparent huge page is escaped from being
isolated in isolate_migratepages_block(). The transparent huge page
can't be migrated and the corresponding memory block can't be put
into offline state.
Fix it by replacing page_mapcount() with total_mapcount(). With this,
the transparent huge page can be isolated and migrated, and the memory
block can be put into offline state. Besides, The page's refcount is
increased a bit earlier to avoid the page is released when the check
is executed.
Did you look into handling pages that are in the swapcache case as well?
See is_refcount_suitable() in mm/khugepaged.c.
Should be easy to reproduce, let me know if you need inspiration.
Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.
A simple reproducer would be (on a system with ordinary swap (not zram))
1) mmap a region (MAP_ANON|MAP_PRIVATE) that can hold a THP
2) Enable THP for that region (MADV_HUGEPAGE)
3) Populate a THP (e.g., write access)
4) PTE-map the THP, for example, using MADV_FREE on the last subpage
5) Trigger swapout of the THP, for example, using MADV_PAGEOUT
6) Read-access to some subpages to fault them in from the swapcache
Now you'd have a THP, which
1) Is partially PTE-mapped into the page table
2) Is in the swapcache (each subpage should have one reference from the swapache)
Now we could test, if alloc_contig_range() will still succeed (e.g., using virtio-mem).
Thanks for the details. Step (4) and (5) can be actually combined. To swap part of
the THP (e.g. one sub-page) will force the THP to be split.
I followed your steps in the attached program, there is no issue to do memory hot-remove
through virtio-mem with or without this patch.
# numactl -p 1 testsuite mm swap -k
Any key to split THP
Any key to swap sub-pages
Any key to read the swapped sub-pages
Page[000]: 0xffffffffffffffff
Page[001]: 0xffffffffffffffff
:
Page[255]: 0xffffffffffffffff
Any key to exit // hold here and the program doesn't exit
(qemu) qom-set vm1 requested-size 0
[ 356.005396] virtio_mem virtio1: plugged size: 0x40000000
[ 356.005996] virtio_mem virtio1: requested size: 0x0
[ 356.350299] Fallback order for Node 0: 0 1
[ 356.350810] Fallback order for Node 1: 1 0
[ 356.351260] Built 2 zonelists, mobility grouping on. Total pages: 491343
[ 356.351998] Policy zone: DMA
Thanks,
Gavin
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <errno.h>
#include "inc/testsuite.h"
struct swap_struct {
int page_size;
int fd;
int flags;
void *buf;
unsigned long len;
int key_break;
};
#define SWAP_DEFAULT_SIZE 0x200000 /* 2MB */
#define SWAP_PAGE_TO_SPLIT 511
#define SWAP_PAGE_TO_SWAP 1
#define SWAP_PAGE_TO_SWAP_NUM 256
#ifndef MADV_PAGEOUT
#define MADV_PAGEOUT 21
#endif
static void usage(void)
{
fprintf(stdout, "testsuite mm swap -l <size> -k\n");
fprintf(stdout, "\n");
fprintf(stdout, "-l: Length of memory to be mapped\n");
fprintf(stdout, "-w: Length of memory to be copied-on-write\n");
fprintf(stdout, "-k: Stop at various stages\n");
fprintf(stdout, "\n");
}
static int swap_init_data(struct swap_struct *m)
{
m->page_size = getpagesize();
m->fd = -1;
m->flags = (MAP_PRIVATE | MAP_ANONYMOUS);
m->len = SWAP_DEFAULT_SIZE;
m->key_break = 0;
return 0;
}
static int swap_handler(int argc, char **argv)
{
struct swap_struct m;
unsigned long *pval;
int i, opt, ret;
ret = swap_init_data(&m);
if (ret)
return ret;
while ((opt = getopt(argc, argv, "l:w:kh")) != -1) {
switch (opt) {
case 'l':
m.len = util_memory_parse_size(optarg);
if (m.len <= SWAP_DEFAULT_SIZE) {
fprintf(stderr, "%s: length 0x%lx less than 0x%x\n",
__func__, m.len, SWAP_DEFAULT_SIZE);
return -1;
}
break;
case 'k':
m.key_break = 1;
break;
case 'h':
usage();
return 0;
}
}
/*
* Setup the area. The area should be backed up with huge pages
* if it suits. Write to the area to ensure the area is populted
* completely.
*/
m.buf = mmap(NULL, m.len, PROT_READ | PROT_WRITE, m.flags, m.fd, 0);
if (m.buf == (void *)-1) {
fprintf(stderr, "Unable do mmap()\n");
goto out;
}
memset(m.buf, 0xff, m.len);
/* Force to split the huge page */
util_misc_key_press(m.key_break, " ", "Any key to split THP");
ret = madvise(m.buf + SWAP_PAGE_TO_SPLIT * m.page_size,
m.page_size, MADV_FREE);
if (ret) {
fprintf(stderr, "Error %d to split THP\n", ret);
goto out;
}
/* Swap one sub-page */
util_misc_key_press(m.key_break, " ", "Any key to swap sub-pages");
ret = madvise(m.buf + SWAP_PAGE_TO_SWAP * m.page_size,
SWAP_PAGE_TO_SWAP_NUM * m.page_size,
MADV_PAGEOUT);
if (ret) {
fprintf(stderr, "Error %d to swap one sub-page\n", ret);
goto out;
}
/* Read the swapped sub-page */
util_misc_key_press(m.key_break, " ", "Any key to read the swapped sub-pages");
for (i = 0; i < SWAP_PAGE_TO_SWAP_NUM; i++) {
pval = (unsigned long *)(m.buf + (SWAP_PAGE_TO_SWAP + i) * m.page_size);
fprintf(stdout, " Page[%03d]: 0x%016lx\n", i, *pval);
}
/* Exit */
util_misc_key_press(m.key_break, " ", "Any key to exit");
out:
if (m.buf != (void *)-1)
munmap(m.buf, m.len);
if (m.fd > 0)
close(m.fd);
return 0;
}
static struct command swap_command = {
.name = "swap",
.handler = swap_handler,
.children = LIST_HEAD_INIT(swap_command.children),
.link = LIST_HEAD_INIT(swap_command.link),
};
int mm_swap_init(void)
{
return command_add(&mm_command, &swap_command);
}
