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

[Bagas Sanjaya <bagasdotme@gmail.com>]

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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?

> 
> 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...

-- 
An old man doll... just what I always wanted! - Clara

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