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From:   David Hildenbrand <david@redhat.com>
To:     Peter Xu <peterx@redhat.com>
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        Andrew Morton <akpm@linux-foundation.org>,
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 <41444eb8-8bb8-8d5b-4cec-be7fa7530d0e@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
Message-ID: <4d8e6f55-66a6-d701-6a94-79f5e2b23e46@redhat.com>
Date:   Fri, 19 Feb 2021 20:14:45 +0100
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>> It's interesting to know about commit 1e356fc14be ("mem-prealloc: reduce large
>> guest start-up and migration time.", 2017-03-14).  It seems for speeding up VM
>> boot, but what I can't understand is why it would cause the delay of hugetlb
>> accounting - I thought we'd fail even earlier at either fallocate() on the
>> hugetlb file (when we use /dev/hugepages) or on mmap() of the memfd which
>> contains the huge pages.  See hugetlb_reserve_pages() and its callers.  Or did
>> I miss something?
> 
> We should fail on mmap() when the reservation happens (unless
> MAP_NORESERVE is passed) I think.
> 
>>
>> I think there's a special case if QEMU fork() with a MAP_PRIVATE hugetlbfs
>> mapping, that could cause the memory accouting to be delayed until COW happens.
> 
> That would be kind of weird. I'd assume the reservation gets properly
> done during fork() - just like for VM_ACCOUNT.
> 
>> However that's definitely not the case for QEMU since QEMU won't work at all as
>> late as that point.
>>
>> IOW, for hugetlbfs I don't know why we need to populate the pages at all if we
>> simply want to know "whether we do still have enough space"..  And IIUC 2)
>> above is the major issue you'd like to solve too.
> 
> To avoid page faults at runtime on access I think. Reservation <=
> Preallocation.

I just learned that there is more to it: (test done on v5.9)

# echo 512 > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
# cat /sys/devices/system/node/node*/meminfo | grep HugePages_
Node 0 HugePages_Total:   512
Node 0 HugePages_Free:    512
Node 0 HugePages_Surp:      0
Node 1 HugePages_Total:     0
Node 1 HugePages_Free:      0
Node 1 HugePages_Surp:      0
# cat /proc/meminfo  | grep HugePages_
HugePages_Total:     512
HugePages_Free:      512
HugePages_Rsvd:        0
HugePages_Surp:        0

# /usr/libexec/qemu-kvm -m 1G -smp 1 -object memory-backend-memfd,id=mem0,size=1G,hugetlb=on,hugetlbsize=2M,policy=bind,host-nodes=0 -numa node,nodeid=0,memdev=mem0 -hda Fedora-Cloud-Base-Rawhide-20201004.n.1.x86_64.qcow2 -nographic
-> works just fine

# /usr/libexec/qemu-kvm -m 1G -smp 1 -object memory-backend-memfd,id=mem0,size=1G,hugetlb=on,hugetlbsize=2M,policy=bind,host-nodes=1 -numa node,nodeid=0,memdev=mem0 -hda Fedora-Cloud-Base-Rawhide-20201004.n.1.x86_64.qcow2 -nographic
-> Does not fail nicely but crashes!


See https://bugzilla.redhat.com/show_bug.cgi?id=1686261 for something similar, however, it no longer applies like that on more recent kernels.

Hugetlbfs reservations don't always protect you (especially with NUMA) - that's why e.g., libvirt always tells QEMU to prealloc.

I think the "issue" is that the reservation happens on mmap(). mbind() runs afterwards. Preallocation saves you from that.

I suspect something similar will happen with anonymous memory with mbind() even if we reserved swap space. Did not test yet, though.

-- 
Thanks,

David / dhildenb