[libvirt] [PATCH 3/5] qemu: prefer memfd for anonymous memory
Dr. David Alan Gilbert
dgilbert at redhat.com
Thu Sep 13 14:36:38 UTC 2018
* Igor Mammedov (imammedo at redhat.com) wrote:
> On Tue, 11 Sep 2018 17:30:31 +0400
> Marc-André Lureau <marcandre.lureau at redhat.com> wrote:
>
> > Hi
> >
> > On Tue, Sep 11, 2018 at 5:14 PM, Igor Mammedov <imammedo at redhat.com> wrote:
> > > On Tue, 11 Sep 2018 12:49:12 +0100
> > > "Dr. David Alan Gilbert" <dgilbert at redhat.com> wrote:
> > >
> > >> * Marc-André Lureau (marcandre.lureau at redhat.com) wrote:
> > >> > Hi
> > >> >
> > >> > On Tue, Sep 11, 2018 at 3:26 PM, Dr. David Alan Gilbert
> > >> > <dgilbert at redhat.com> wrote:
> > >> > > * Marc-André Lureau (marcandre.lureau at redhat.com) wrote:
> > >> > >> Hi
> > >> > >>
> > >> > >> On Tue, Sep 11, 2018 at 2:32 PM, Dr. David Alan Gilbert
> > >> > >> <dgilbert at redhat.com> wrote:
> > >> > >> > * Marc-André Lureau (marcandre.lureau at redhat.com) wrote:
> > >> > >> >> Hi
> > >> > >> >>
> > >> > >> >> On Tue, Sep 11, 2018 at 12:37 PM, Michal Privoznik <mprivozn at redhat.com> wrote:
> > >> > >> >> > On 09/11/2018 12:46 AM, John Ferlan wrote:
> > >> > >> >> >>
> > >> > >> >> >> On 09/07/2018 07:32 AM, marcandre.lureau at redhat.com wrote:
> > >> > >> >> >>> From: Marc-André Lureau <marcandre.lureau at redhat.com>
> > >> > >> >> >>>
> > >> > >> >> >>
> > >> > >> >> >> Would be nice to have a few more words here. If you provide them I can
> > >> > >> >> >> add them... The if statement is difficult to read unless you know what
> > >> > >> >> >> each field really means.
> > >> > >> >> >>
> > >> > >> >> >> secondary question - should we document what gets used?, e.g.:
> > >> > >> >> >>
> > >> > >> >> >> https://libvirt.org/formatdomain.html#elementsMemoryBacking
> > >> > >> >> >>
> > >> > >> >> >> Seems to me the preference to use memfd is for memory backing using
> > >> > >> >> >> anonymous source for nvdimm's without a defined path, but sometimes my
> > >> > >> >> >> wording doesn't match reality.
> > >> > >> >> >
> > >> > >> >> > I don't think we want to tell users what backend are we going to use
> > >> > >> >> > under what conditions. Firstly, these conditions will change (as they
> > >> > >> >> > did in the past). Secondly, what backend libvirt decides to use is no
> > >> > >> >> > business of users. I mean, they care about providing XML that matches
> > >> > >> >> > their demands. It's libvirt's job to fulfil them.
> > >> > >> >> >
> > >> > >> >> > Look at this from the other way: if an user wants to have
> > >> > >> >> > memory-backend-file for his domain, how would they enforce it once memfd
> > >> > >> >> > is merged? Sure, they can tweak their memoryBacking settings, but that
> > >> > >> >> > would work only until we decide to change the decision process for mem
> > >> > >> >> > backend.
> > >> > >> >> >
> > >> > >> >> > What I am more worried about is migration. What happens if I migrate a
> > >> > >> >> > hugepages domain from older libvirt to a newer one (the former doesn't
> > >> > >> >> > support memfd, the latter does). On the source the domain was started
> > >> > >> >> > with memory-backend-file (or memory-backend-ram with -mem-path). And
> > >> > >> >> > during migration, the generated cmd line would use memfd. And I don't
> > >> > >> >> > think qemu is capable of dealing with this discrepancy, is it?
> > >> > >> >>
> > >> > >> >>
> > >> > >> >> Actually, qemu doesn't care about the hostmem backend kind, it should
> > >> > >> >> handle the migration ok.
> > >> > >> >>
> > >> > >> >> However, there seems to be a bug in qemu, and hostmem backend don't
> > >> > >> >> use the right qom object name.
> > >> > >> >
> > >> > >> > Can you give me the command lines you're using?
> > >> > >>
> > >> > >> qemu -m 4096 -object memory-backend-ram,id=mem,size=4G -numa
> > >> > >> node,memdev=mem -monitor stdio
> > >> > >> qemu -m 4096 -object
> > >> > >> memory-backend-file,id=mem,size=4G,mem-path=/tmp/foo -numa
> > >> > >> node,memdev=mem -monitor stdio
> > >> > >> qemu -m 4096 -object memory-backend-memfd,id=mem,size=4G -numa
> > >> > >> node,memdev=mem -monitor stdio
> > >> > >
> > >> > > There seem to be two different problems (at least); there's that
> > >> > > escaping problem where the /'s are shown as \x2f in into qom-tree,
> > >> >
> > >> > That's not a problem, this is done in memory_region_escape_name()
> > >> >
> > >> > > but info ramblock looks saner, but is still showing the difference:
> > >> > >
> > >> > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-ram,id=mem,size=1G -numa node,memdev=mem -monitor stdio
> > >> > > (qemu) info ramblock
> > >> > > Block Name PSize Offset Used Total
> > >> > > mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000
> > >> > >
> > >> > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-file,id=mem,size=1G,mem-path=/tmp/foo -numa node,memdev=mem -monitor stdio
> > >> > > (qemu) info ramblock
> > >> > > Block Name PSize Offset Used Total
> > >> > > /objects/mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000
> > >> > >
> > >> > > ./x86_64-softmmu/qemu-system-x86_64 -m 1024 -object memory-backend-memfd,id=mem,size=1G -numa node,memdev=mem -monitor stdio
> > >> > > QEMU 3.0.50 monitor - type 'help' for more information
> > >> > > (qemu) info ramblock
> > >> > > Block Name PSize Offset Used Total
> > >> > > /objects/mem 4 KiB 0x0000000000000000 0x0000000040000000 0x0000000040000000
> > >> > >
> > >> > > hostmem-file.c is using object_get_canonical_path to get the RAMBlock
> > >> > > where as hostmem-ram.c is using object_get_canonical_path_**component**
> > >> > >
> > >> > > The problem is if we change either of them then again we break
> > >> > > migration compatibility.
> > >> >
> > >> > Yes, that was the object of my question :)
> > >> >
> > >> > > We could wire it to a machine type and/or property, so that
> > >> > > memory-backend-ram would use the long name on newere qemus with an
> > >> > > appropriate flag?
> > >> >
> > >> > Good idea, I can prepare a patch.
> > >>
> > >> Great; if you add the property to use the longname, then turn that
> > >> property on in the newer machine type it should work. A qemu that has
> > >> the property can then be assumed to the right thing when set.
> > > compat properties mechanism is applicable only for device based objects
> > > and backends are not based on it. So it won't be so easy, one basically
> > > would need to re-implement or event better extend compat props mechanism
> > > to backends.
> > >
> >
> > indeed
> >
> > >
> > >> > However, libvirt will have to learn of this migration issue with older
> > >> > version, it's probably not worth to try to make more workarounds.
> > >>
> > >> Yeh I'm not sure what your heuristics look like for these choices.
> > >> But for a VM without this fix then you can't convert from backend-ram to
> > >> memfd.
> > > I wouldn't try migrate from one to backend type to another automatically
> > > if domain used backend-ram than libvirt should start target with the same
> > > backend (it not only ram block name in migration stream, but could also
> > > involve ramblock's alignment, padding, guard pages or something else as
> > > it's different backends and potentially can change its default behavior
> > > independently from each other).
> >
> > Then libvirt can't transparently use memfd, and we will go back to my
> > initial suggestion to have a new memory backing source kind in the
> > domain XML named "memfd".
> less magic the better, the only downside is that implementation
> details of a QEMU backend sip through abstraction libvirt is
> supposed to produce for it's users and a question how users are
> supposed to pick a backend variant for their needs.
>
> >
> > Are "ramblock's alignment, padding, guard pages" exposed in domain
> > XML? Didn't they change over time in qemu wtihout libvirt noticing?
> > Why allocation with memfd couldn't be transparently be changed the
> > same way?
> >
> > > Redefining meaning of 'anonymous' from backend-ram to memfd is fine only
> > > if libvirt is able to distinguish old domains with ram backend vs memfd
> > > (so it could start domains accordingly, i.e. no cross migration).
> >
> > And memory-backend-file used as anonymous memory (without explicit path etc).
> >
> > > Otherwise we would be creating time bomb, that would explode
> > > when 2 independent backends change in incompatible manner.
> >
> > If there is such a limitation, qemu should prevent it then. It seems
> > qemu let you migrate from/to the various hostmem-* (as long they use
> > the same name, which is the case for -file and -memfd at this point).
> > Why restrict that now?
> it works buy luck not by design. Even though qemu doesn't block it,
> it doesn't mean that's the right thing to do.
> Rule of the thumb with migration is that CLI on destination should
> match one one source (i.e. no magical cli replacements).
> If it's not then user is to blame.
The rule isn't actually that strong. We normally allow the backends to
change as long as the guest visible parts don't. For example, it's
perfectly legal to migrate between a qemu that's got it's virtio-blk
wired to a NFS disk to a qemu that's got it wired to iSCSI - the guest
view in the two cases is the same but the command line is quite
different. Similarly for networking you can flip to different tap
setups.
So as long as the change:
a) looks identical to the guest
b) Doesn't have any backend specific migration data
then a migration should work and I'd expect it to work.
Dave
> >
> > >
> > >> Dave
> > >>
> > >> >
> > >> > > Dave
> > >> > >
> > >> > >
> > >> > >
> > >> > >> >
> > >> > >> > Dave
> > >> > >> >
> > >> > >> >> with memory-backend-ram:
> > >> > >> >>
> > >> > >> >> (qemu) info qom-tree /objects
> > >> > >> >> /objects (container)
> > >> > >> >> /mem (memory-backend-file)
> > >> > >> >> /mem[0] (qemu:memory-region)
> > >> > >> >>
> > >> > >> >> But with memory-backend-file or memory-backend-memfd:
> > >> > >> >>
> > >> > >> >> (qemu) info qom-tree /objects
> > >> > >> >> /objects (container)
> > >> > >> >> /mem (memory-backend-file)
> > >> > >> >> /\x2fobjects\x2fmem[0] (qemu:memory-region)
> > >> > >> >>
> > >> > >> >>
> > >> > >> >> This causes migration to fail because of the object naming mismatch.
> > >> > >> >>
> > >> > >> >> It can migrate from/to -file and -memfd, since they use the same
> > >> > >> >> "broken" name, but not with -ram.
> > >> > >> >>
> > >> > >> >> I don't know how we can solve this migration issue without breaking
> > >> > >> >> things further. Any idea David?
> > >> > >> >>
> > >> > >> >> > Or is memfd going to be used only for hugepages + <source
> > >> > >> >> > type='anonymous'/> case (which is not allowed now and thus migration
> > >> > >> >> > scenario I'm describing can't happen)?
> > >> > >> >>
> > >> > >> >> With those patches, memfd is used for anonymous memory (shared or not,
> > >> > >> >> hpt or not) with an explicit numa configuration.
> > >> > >> >>
> > >> > >> >> thanks
> > >> > >> > --
> > >> > >> > Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> > >> > > --
> > >> > > Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> > >> --
> > >> Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
> > >
>
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
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