Re: [PATCH v2 8/9] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE

[David Hildenbrand]

On 05.03.21 16:51, Peter Xu wrote:
> On Fri, Mar 05, 2021 at 04:44:36PM +0100, David Hildenbrand wrote:
> > On 05.03.21 16:42, Peter Xu wrote:
> > > On Fri, Mar 05, 2021 at 11:16:33AM +0100, David Hildenbrand wrote:
> > > > +#define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
> > > > +static bool map_noreserve_effective(int fd, bool readonly, bool shared)
> > > > +{
> > >
> > > [...]
> > >
> > > > @@ -184,8 +251,7 @@ void *qemu_ram_mmap(int fd,
> > > >        size_t offset, total;
> > > >        void *ptr, *guardptr;
> > > > -    if (noreserve) {
> > > > -        error_report("Skipping reservation of swap space is not supported");
> > > > +    if (noreserve && !map_noreserve_effective(fd, shared, readonly)) {
> > >
> > > Need to switch "shared" & "readonly"?
> >
> > Indeed, interestingly it has the same effect (as we don't have anonymous
> > read-only memory in QEMU :) )
>
> But note there is still a "g_assert(!shared || fd >= 0);" inside.. :)

Aaaaaand, I just figured that we actually can create shared anonymous 
memory in QEMU, simply via

-object memory-backend-ram,share=on

Introduced in 06329ccecfa0 ("mem: add share parameter to 
memory-backend-ram"). That's also where we introduced the "shared" flag 
for qemu_anon_ram_alloc().

That commit mentions a use case for "RDMA devices in order to remap 
non-contiguous QEMU virtual addresses to a contiguous virtual address 
range.". I fail to understand why that requires sharing RAM with child 
processes.

Especially:

a) qemu_ram_is_shared() returned false before patch #1. RAM_SHARED is 
never set.

b) qemu_ram_remap() does not work as expected?

c) ram_discard_range() is broken with shared anonymous memory. Instead 
of MADV_DONTNEED we need MADV_REMOVE.

This looks like a partially broken feature and I wonder if there is an 
actual user.

@Marcel, can you clarify if there is an actual use case for shared 
anonymous memory in QEMU? I.e., if the original use case that required 
that change is valid? (and why it wasn't able to just use proper shmem)

Shared anonymous memory is weird.

--
Thanks,

David / dhildenb