[PATCH v8 0/7] Make alloc_contig_range handle Hugetlb pages

[Oscar Salvador]

So, after Mike's work [1] has gone in, here is a new version of top.

NOTE: If you are going to try out this patchset, be aware of [2].
      You should fix that up before any testing.

[1] https://patchwork.kernel.org/project/linux-mm/cover/20210409205254.242291-1-mike.kravetz@xxxxxxxxxx/
[2] https://patchwork.kernel.org/project/linux-mm/patch/1617259448-22529-5-git-send-email-anshuman.khandual@xxxxxxx/#24108247

v7 -> v8:
 - Remove clearing of flag from prep_new_huge_page()
 - Reworded comment in alloc_and_dissolve_huge_page() by Michal
 - Adressed feedback from David
 - Collect Reviewed-by/Acked-by

v6 -> v7:
 - Add patch to move the clearing of HPageFreed flag out of the lock
 - Add patch to decouple counter handling from prep_new_huge_page().
   We end up with two new functions, __prep_new_huge_page() which
   does the proper initialization of the new huge page, and
   __prep_account_new_huge_page(), which increments hstate->nr_huge_pages.
   prep_new_huge_page() still calls both of them (details in patch#4).
   This comes in handy in patch#5, where the whole operation of
   replacing the page must be done under the lock.
 - Remove Reviewed-by/Acked-by from patch#5, as needs to be checked again.

v5 -> v6:
 - Collect Acked-by from Michal
 - Adressed feedback for patch#2 (expand the comment about migrate_pfn and
   change return values)
 - Complete pathc#3's changelog (per Michal)
 - Place retry lock inside of alloc_and_dissolve_huge_page()

v4 -> v5:
 - Collect Acked-by and Reviewed-by from David and Vlastimil
 - Drop racy checks in pfn_range_valid_contig (David)
 - Rebased on top of 5.12-rc3

v3 -> v4:
 - Addressed some feedback from David and Michal
 - Make more clear what hugetlb_lock protects in isolate_or_dissolve_huge_page
 - Start reporting proper error codes from isolate_migratepages_{range,block}
 - Bail out earlier in __alloc_contig_migrate_range on -ENOMEM
 - Addressed internal feedback from Vastlimil wrt. compaction code changes

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.

 Since we also need to start reporting errors down the chain (e.g: -ENOMEM due to
 not be able to allocate a new hugetlb page), isolate_migratepages_{range,block}
 interfaces  need to change to start reporting error codes instead of the pfn == 0
 vs pfn != 0 scheme it is using right now.
 From now on, isolate_migratepages_block will not return the next pfn to be scanned
 anymore, but -EINTR, -ENOMEM or 0, so we the next pfn to be scanned will be recorded
 in cc->migrate_pfn field (as it is already done in isolate_migratepages_range()).

 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"

 And then with this patchset running:

 "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"

Oscar Salvador (7):
  mm,page_alloc: Bail out earlier on -ENOMEM in
    alloc_contig_migrate_range
  mm,compaction: Let isolate_migratepages_{range,block} return error
    codes
  mm,hugetlb: Drop clearing of flag from prep_new_huge_page
  mm,hugetlb: Split prep_new_huge_page functionality
  mm: Make alloc_contig_range handle free hugetlb pages
  mm: Make alloc_contig_range handle in-use hugetlb pages
  mm,page_alloc: Drop unnecessary checks from pfn_range_valid_contig

 include/linux/hugetlb.h |   7 +++
 mm/compaction.c         |  93 +++++++++++++++++++++---------
 mm/hugetlb.c            | 150 ++++++++++++++++++++++++++++++++++++++++++++++--
 mm/internal.h           |  10 +++-
 mm/page_alloc.c         |  22 +++----
 mm/vmscan.c             |   5 +-
 6 files changed, 240 insertions(+), 47 deletions(-)

-- 
2.16.3