Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[Gavin Shan <gshan@redhat.com>]

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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

[-- Attachment #2: swap.c --]
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#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);
}