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