Re: [libvirt] [PATCH 3/5] qemu: prefer memfd for anonymous memory

Hi On Tue, Sep 11, 2018 at 3:26 PM, Dr. David Alan Gilbert <dgilbert(a)redhat.com&gt; wrote: > * Marc-André Lureau (marcandre.lureau(a)redhat.com) wrote: >> Hi >> >> On Tue, Sep 11, 2018 at 2:32 PM, Dr. David Alan Gilbert >> <dgilbert(a)redhat.com&gt; wrote: >> > * Marc-André Lureau (marcandre.lureau(a)redhat.com) wrote: >> >> Hi >> >> >> >> On Tue, Sep 11, 2018 at 12:37 PM, Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: >> >> > On 09/11/2018 12:46 AM, John Ferlan wrote: >> >> >> >> >> >> On 09/07/2018 07:32 AM, marcandre.lureau(a)redhat.com wrote: >> >> >>> From: Marc-André Lureau <marcandre.lureau(a)redhat.com&gt; >> >> >>> >> >> >> >> >> >> 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.

However, libvirt will have to learn of this migration issue with older version, it's probably not worth to try to make more workarounds.

> 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(a)redhat.com / Manchester, UK > -- > Dr. David Alan Gilbert / dgilbert(a)redhat.com / Manchester, UK