Re: [PATCH v7 09/15] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux

[Daniel P . Berrangé]

On Tue, May 04, 2021 at 12:21:25PM +0200, David Hildenbrand wrote:
> On 04.05.21 12:09, Daniel P. Berrangé wrote:
> > On Wed, Apr 28, 2021 at 03:37:48PM +0200, David Hildenbrand wrote:
> > > Let's support RAM_NORESERVE via MAP_NORESERVE on Linux. The flag has no
> > > effect on most shared mappings - except for hugetlbfs and anonymous 
> > > memory.
> > > 
> > > Linux man page:
> > >    "MAP_NORESERVE: Do not reserve swap space for this mapping. When swap
> > >    space is reserved, one has the guarantee that it is possible to modify
> > >    the mapping. When swap space is not reserved one might get SIGSEGV
> > >    upon a write if no physical memory is available. See also the 
> > > discussion
> > >    of the file /proc/sys/vm/overcommit_memory in proc(5). In kernels 
> > > before
> > >    2.6, this flag had effect only for private writable mappings."
> > > 
> > > Note that the "guarantee" part is wrong with memory overcommit in Linux.
> > > 
> > > Also, in Linux hugetlbfs is treated differently - we configure reservation
> > > of huge pages from the pool, not reservation of swap space (huge pages
> > > cannot be swapped).
> > > 
> > > The rough behavior is [1]:
> > > a) !Hugetlbfs:
> > > 
> > >    1) Without MAP_NORESERVE *or* with memory overcommit under Linux
> > >       disabled ("/proc/sys/vm/overcommit_memory == 2"), the following
> > >       accounting/reservation happens:
> > >        For a file backed map
> > >         SHARED or READ-only - 0 cost (the file is the map not swap)
> > >         PRIVATE WRITABLE - size of mapping per instance
> > > 
> > >        For an anonymous or /dev/zero map
> > >         SHARED   - size of mapping
> > >         PRIVATE READ-only - 0 cost (but of little use)
> > >         PRIVATE WRITABLE - size of mapping per instance
> > > 
> > >    2) With MAP_NORESERVE, no accounting/reservation happens.
> > > 
> > > b) Hugetlbfs:
> > > 
> > >    1) Without MAP_NORESERVE, huge pages are reserved.
> > > 
> > >    2) With MAP_NORESERVE, no huge pages are reserved.
> > > 
> > > Note: With "/proc/sys/vm/overcommit_memory == 0", we were already able
> > > to configure it for !hugetlbfs globally; this toggle now allows
> > > configuring it more fine-grained, not for the whole system.
> > > 
> > > The target use case is virtio-mem, which dynamically exposes memory
> > > inside a large, sparse memory area to the VM.
> > 
> > Can you explain this use case in more real world terms, as I'm not
> > understanding what a mgmt app would actually do with this in
> > practice ?
> 
> Let's consider huge pages for simplicity. Assume you have 128 free huge
> pages in your hypervisor that you want to dynamically assign to VMs.
> 
> Further assume you have two VMs running. A workflow could look like
> 
> 1. Assign all huge pages to VM 0
> 2. Reassign 64 huge pages to VM 1
> 3. Reassign another 32 huge pages to VM 1
> 4. Reasssign 16 huge pages to VM 0
> 5. ...
> 
> Basically what we're used to doing with "ordinary" memory.

What does this look like in terms of the memory backend configuration
when you boot VM 0 and VM 1 ?

Are you saying that we boot both VMs with

   -object hostmem-memfd,size=128G,hugetlb=yes,hugetlbsize=1G,reserve=off

and then we have another property set on 'virtio-mem' to tell it
how much/little of that 128 G, to actually give to the guest ?
How do we change that at runtime ?


> For that to work with virtio-mem, you'll have to disable reservation of huge
> pages for the virtio-mem managed memory region.
> 
> (prealloction of huge pages in virtio-mem to protect from user mistakes is a
> separate work item)
> 
> reserve=off will be the default for virtio-mem, and actual
> reservation/preallcoation will be done within virtio-mem. There could be use
> for "reserve=off" for virtio-balloon use cases as well, but I'd like to
> exclude that from the discussion for now.

The hostmem backend defaults are indepdant of frontend usage, so when you
say reserve=off is the default for virtio-mem, are you expecting the mgmt
app like libvirt to specify that ?

Regards,
Daniel
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