BUG: MADV_COLLAPSE doesn't work for XFS files

[Ryan Roberts <ryan.roberts@xxxxxxx>]

Hi all,

I've just noticed that when applied to a file mapping for a file on xfs, MADV_COLLAPSE returns EINVAL. The same test case works fine if the file is on ext4. 

I think the root cause is that the implementation bails out if it finds a (non-PMD-sized) large folio in the page cache for any part of the file covered by the region. XFS does readahead into large folios so we hit this issue. See khugepaged.h:collapse_file():

		if (PageTransCompound(page)) {
			struct page *head = compound_head(page);

			result = compound_order(head) == HPAGE_PMD_ORDER &&
					head->index == start
					/* Maybe PMD-mapped */
					? SCAN_PTE_MAPPED_HUGEPAGE
					: SCAN_PAGE_COMPOUND;
			goto out_unlock;
		}

I'm not sure if this is already a known issue? I don't have time to work on a fix for this right now, so thought I would highlight it at least. I might get around to it at some point in the future if nobody else tackles it.

Thanks,
Ryan


Test case I've been using:

-->8--

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

#ifndef MADV_COLLAPSE
#define MADV_COLLAPSE		25
#endif

#define handle_error(msg) 	do { perror(msg); exit(EXIT_FAILURE); } while (0)

#define SZ_1K			1024
#define SZ_1M			(SZ_1K * SZ_1K)
#define ALIGN(val, align)	(((val) + ((align) - 1)) & ~((align) - 1))

#if 1
// ext4
#define DATA_FILE		"/home/ubuntu/data.txt"
#else
// xfs
#define DATA_FILE		"/boot/data.txt"
#endif

int main(void)
{
	int fd;
	char *mem;
	int ret;

	fd = open(DATA_FILE, O_RDONLY);
	if (fd == -1)
        	handle_error("open");

	mem = mmap(NULL, SZ_1M * 4, PROT_READ | PROT_EXEC, MAP_PRIVATE, fd, 0);
	close(fd);
	if (mem == MAP_FAILED)
        	handle_error("mmap");

	printf("1: pid=%d, mem=%p\n", getpid(), mem);
	getchar();

	mem = (char *)ALIGN((unsigned long)mem, SZ_1M * 2);
	ret = madvise(mem, SZ_1M * 2, MADV_COLLAPSE);
	if (ret)
		handle_error("madvise");

	printf("2: pid=%d, mem=%p\n", getpid(), mem);
	getchar();

	return 0;
}

-->8--