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.
>
