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Date: Mon, 15 Mar 2021 10:06:40 +0100
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>,
Muchun Song <songmuchun@...edance.com>,
Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 0/4] Make alloc_contig_range handle Hugetlb pages
On 10.03.21 16:08, 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@...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"
>
> 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(-)
>
BTW, I stumbled yesterday over
alloc_contig_pages()->pfn_range_valid_contig():
if (page_count(page) > 0)
rerurn false;
if (PageHuge(page))
return false;
As used by memtrace and for gigantic pages. We can now
a) Drop these check completely, as it's best-effort only and racy.
alloc_contig_pages()/alloc_contig_range() will handle it properly.
b) Similarly, check for gigantic pages and/or movability/migratability.
Dropping both checks might be the right thing to do: might significantly
increase allocation chances -- as we actually end up migrating busy
pages ...
--
Thanks,
David / dhildenb
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