Re: [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages

David Hildenbrand
2021-03-01T12:44:53Z

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