Re: BUG: MADV_COLLAPSE doesn't work for XFS files

[Ryan Roberts]

On 28/09/2023 11:54, Bagas Sanjaya wrote:
> On Thu, Sep 28, 2023 at 10:55:17AM +0100, Ryan Roberts wrote:
>> 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 don't see any hint to -EINVAL above. Am I missing something?

The SCAN_PAGE_COMPOUND result ends up back at madvise_collapse() where it
eventually gets converted to -EINVAL by madvise_collapse_errno().

> 
>>
>> 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--
>>
> 
> Confused...

This is a user space test case that shows the problem; data.txt needs to be at
least 4MB and on a mounted ext4 and xfs filesystem. By toggling the '#if 1' to
0, you can see the different behaviours for ext4 and xfs -
handle_error("madvise") fires with EINVAL in the xfs case. The getchar()s are
leftovers from me looking at the smaps file.

>