linux-kernel - Re: [PATCH RFC] mm/madvise: introduce MADV

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Date:   Fri, 19 Feb 2021 20:14:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>, Jann Horn <jannh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Hansen <dave.hansen@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory

>> 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