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

[Marcel Apfelbaum <mapfelba@redhat.com>]

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Hi David,

On Sun, Mar 7, 2021 at 3:18 PM David Hildenbrand <david@redhat.com> wrote:

> 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)
>
>
As you correctly stated, the PVRDMA device requires remapping of
non-contiguous QEMU
virtual addresses to a contiguous virtual address range.

In order to do so it calls
     mremap (... , MREMAP_MAYMOVE | MREMAP_FIXED, ...)
The above call succeeds only if the memory is marked as "shared".


> Shared anonymous memory is weird.
>

In this case it is not about sharing the memory between different
processes, but about being
able to remap it.

Thanks,
Marcel


>
> --
> Thanks,
>
> David / dhildenb
>
>

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