linux-kernel - Re: [PATCH v3 0/2] Make alloc_contig

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Date:   Mon, 1 Mar 2021 13:43:00 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages

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@...alhost ~]# 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