Re: [BUG/RFC] mm/madvise: MADV_WILLNEED skips swapped file-backed COW pages

[zhaozhengzhuo]

Hi Mike,

Thanks for the report. I reproduced this on x86-64 with the current
mm-unstable tree at:

  e3b5239afe1b ("mm: gup: cleanup the gup_fast_*() call chain")

The test used a 64 MiB ordinary file-backed MAP_PRIVATE mapping and
disk-backed swap. Since the test machine has 128 GiB of RAM, the pages
initially remained in swap cache after MADV_PAGEOUT. I therefore added
controlled memory pressure between MADV_PAGEOUT and MADV_WILLNEED so
that the swap-cache folios were actually reclaimed.

Across three runs, the pswpin deltas (in 4 KiB pages) were:

                        run 1     run 2     run 3
  after MADV_WILLNEED       2         0         0
  during subsequent touch 16327     15811     15676

The mapping still reported about 64 MiB in Swap after MADV_WILLNEED,
and almost all swap reads occurred only when the pages were touched.

This appears to confirm that madvise_willneed() skips the private swap
entries because the VMA still has vm_file and therefore takes the file
readahead path. vfs_fadvise(POSIX_FADV_WILLNEED) can prefetch the
underlying file contents, but not the modified private contents stored
in swap.

Are you already working on a fix? If testing or other help would be
useful, I would be happy to help.

Thanks,
zhaozhengzhuo