On Tue, Feb 11, 2025 at 12:05 PM Andrew Morton
[off-list ref] wrote:

On Mon, 10 Feb 2025 20:37:42 +0100 David Hildenbrand [off-list ref] wrote:

quoted

Against mm-hotfixes-stable for now.

Discussing the PageTail() call in make_device_exclusive_range() with
Willy, I recently discovered [1] that device-exclusive handling does
not properly work with THP, making the hmm-tests selftests fail if THPs
are enabled on the system.

Looking into more details, I found that hugetlb is not properly fenced,
and I realized that something that was bugging me for longer -- how
device-exclusive entries interact with mapcounts -- completely breaks
migration/swapout/split/hwpoison handling of these folios while they have
device-exclusive PTEs.

The program below can be used to allocate 1 GiB worth of pages and
making them device-exclusive on a kernel with CONFIG_TEST_HMM.

Once they are device-exclusive, these folios cannot get swapped out
(proc$pid/smaps_rollup will always indicate 1 GiB RSS no matter how
much one forces memory reclaim), and when having a memory block onlined
to ZONE_MOVABLE, trying to offline it will loop forever and complain about
failed migration of a page that should be movable.

# echo offline > /sys/devices/system/memory/memory136/state
# echo online_movable > /sys/devices/system/memory/memory136/state
# ./hmm-swap &
... wait until everything is device-exclusive
# echo offline > /sys/devices/system/memory/memory136/state
[  285.193431][T14882] page: refcount:2 mapcount:0 mapping:0000000000000000
  index:0x7f20671f7 pfn:0x442b6a
[  285.196618][T14882] memcg:ffff888179298000
[  285.198085][T14882] anon flags: 0x5fff0000002091c(referenced|uptodate|
  dirty|active|owner_2|swapbacked|node=1|zone=3|lastcpupid=0x7ff)
[  285.201734][T14882] raw: ...
[  285.204464][T14882] raw: ...
[  285.207196][T14882] page dumped because: migration failure
[  285.209072][T14882] page_owner tracks the page as allocated
[  285.210915][T14882] page last allocated via order 0, migratetype
  Movable, gfp_mask 0x140dca(GFP_HIGHUSER_MOVABLE|__GFP_COMP|__GFP_ZERO),
  id 14926, tgid 14926 (hmm-swap), ts 254506295376, free_ts 227402023774
[  285.216765][T14882]  post_alloc_hook+0x197/0x1b0
[  285.218874][T14882]  get_page_from_freelist+0x76e/0x3280
[  285.220864][T14882]  __alloc_frozen_pages_noprof+0x38e/0x2740
[  285.223302][T14882]  alloc_pages_mpol+0x1fc/0x540
[  285.225130][T14882]  folio_alloc_mpol_noprof+0x36/0x340
[  285.227222][T14882]  vma_alloc_folio_noprof+0xee/0x1a0
[  285.229074][T14882]  __handle_mm_fault+0x2b38/0x56a0
[  285.230822][T14882]  handle_mm_fault+0x368/0x9f0
...

This series fixes all issues I found so far.

Cool.

Barry, could you please redo your series "mm: batched unmap lazyfree
large folios during reclamation" on top of this (on top of mm-unstable,
ideally).

Sure. Thanks for letting me know.