On 22.02.21 14:51, Oscar Salvador wrote:

v2 -> v3:
  - Drop usage of high-level generic helpers in favour of
    low-level approach (per Michal)
  - Check for the page to be marked as PageHugeFreed
  - Add a one-time retry in case someone grabbed the free huge page
    from under us

v1 -> v2:
  - Adressed feedback by Michal
  - Restrict the allocation to a node with __GFP_THISNODE
  - Drop PageHuge check in alloc_and_dissolve_huge_page
  - Re-order comments in isolate_or_dissolve_huge_page
  - Extend comment in isolate_migratepages_block
  - Place put_page right after we got the page, otherwise
    dissolve_free_huge_page will fail

  RFC -> v1:
  - Drop RFC
  - Addressed feedback from David and Mike
  - Fence off gigantic pages as there is a cyclic dependency between
    them and alloc_contig_range
  - Re-organize the code to make race-window smaller and to put
    all details in hugetlb code
  - Drop nodemask initialization. First a node will be tried and then we
    will back to other nodes containing memory (N_MEMORY). Details in
    patch#1's changelog
  - Count new page as surplus in case we failed to dissolve the old page
    and the new one. Details in patch#1.

Cover letter:

  alloc_contig_range lacks the hability for handling HugeTLB pages.
  This can be problematic for some users, e.g: CMA and virtio-mem, where those
  users will fail the call if alloc_contig_range ever sees a HugeTLB page, even
  when those pages lay in ZONE_MOVABLE and are free.
  That problem can be easily solved by replacing the page in the free hugepage
  pool.

  In-use HugeTLB are no exception though, as those can be isolated and migrated
  as any other LRU or Movable page.

  This patchset aims for improving alloc_contig_range->isolate_migratepages_block,
  so HugeTLB pages can be recognized and handled.

  Below is an insight from David (thanks), where the problem can clearly be seen:

  "Start a VM with 4G. Hotplug 1G via virtio-mem and online it to
  ZONE_MOVABLE. Allocate 512 huge pages.

  [root@localhost ~]# cat /proc/meminfo
  MemTotal:        5061512 kB
  MemFree:         3319396 kB
  MemAvailable:    3457144 kB
  ...
  HugePages_Total:     512
  HugePages_Free:      512
  HugePages_Rsvd:        0
  HugePages_Surp:        0
  Hugepagesize:       2048 kB


  The huge pages get partially allocate from ZONE_MOVABLE. Try unplugging
  1G via virtio-mem (remember, all ZONE_MOVABLE). Inside the guest:

  [  180.058992] alloc_contig_range: [1b8000, 1c0000) PFNs busy
  [  180.060531] alloc_contig_range: [1b8000, 1c0000) PFNs busy
  [  180.061972] alloc_contig_range: [1b8000, 1c0000) PFNs busy
  [  180.063413] alloc_contig_range: [1b8000, 1c0000) PFNs busy
  [  180.064838] alloc_contig_range: [1b8000, 1c0000) PFNs busy
  [  180.065848] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
  [  180.066794] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
  [  180.067738] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
  [  180.068669] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
  [  180.069598] alloc_contig_range: [1bfc00, 1c0000) PFNs busy"

Same experiment with ZONE_MOVABLE:

a) Free huge pages: all memory can get unplugged again.

b) Allocated/populated but idle huge pages: all memory can get unplugged 
again.

c) Allocated/populated but all 512 huge pages are read/written in a 
loop: all memory can get unplugged again, but I get a single

[  121.192345] alloc_contig_range: [180000, 188000) PFNs busy

Most probably because it happened to try migrating a huge page while it 
was busy. As virtio-mem retries on ZONE_MOVABLE a couple of times, it 
can deal with this temporary failure.



Last but not least, I did something extreme:

]# cat /proc/meminfo
MemTotal:        5061568 kB
MemFree:          186560 kB
MemAvailable:     354524 kB
...
HugePages_Total:    2048
HugePages_Free:     2048
HugePages_Rsvd:        0
HugePages_Surp:        0


Triggering unplug would require to dissolve+alloc - which now fails when 
trying to allocate an additional ~512 huge pages (1G).


As expected, I can properly see memory unplug not fully succeeding. + I 
get a fairly continuous stream of

[  226.611584] alloc_contig_range: [19f400, 19f800) PFNs busy
...

But more importantly, the hugepage count remains stable, as configured 
by the admin (me):

HugePages_Total:    2048
HugePages_Free:     2048
HugePages_Rsvd:        0
HugePages_Surp:        0


-- 
Thanks,

David / dhildenb