Re: [PATCH v4 0/4] Make alloc_contig_range handle Hugetlb pages

[Oscar Salvador <osalvador@suse.de>]

On Wed, Mar 10, 2021 at 04:08:49PM +0100, Oscar Salvador wrote:
> 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 (4):
>   mm,page_alloc: Bail out earlier on -ENOMEM in
>     alloc_contig_migrate_range
>   mm,compaction: Let isolate_migratepages_{range,block} return error
>     codes
>   mm: Make alloc_contig_range handle free hugetlb pages
>   mm: Make alloc_contig_range handle in-use hugetlb pages
> 
>  include/linux/hugetlb.h |   7 +++
>  mm/compaction.c         |  89 ++++++++++++++++++++++++----------
>  mm/hugetlb.c            | 125 +++++++++++++++++++++++++++++++++++++++++++++++-
>  mm/internal.h           |   2 +-
>  mm/page_alloc.c         |  15 ++++--
>  mm/vmscan.c             |   5 +-
>  6 files changed, 209 insertions(+), 34 deletions(-)

Kindly ping :-)


-- 
Oscar Salvador
SUSE L3