On 19.10.23 11:26, Xiaoyao Li wrote:
On 10/19/2023 4:26 PM, David Hildenbrand wrote:
On 18.10.23 18:27, Xiaoyao Li wrote:
On 10/18/2023 5:26 PM, David Hildenbrand wrote:
On 18.10.23 11:02, Xiaoyao Li wrote:
On 10/18/2023 3:42 PM, David Hildenbrand wrote:
On 18.10.23 05:02, Xiaoyao Li wrote:
David,
On 7/6/2023 3:56 PM, David Hildenbrand wrote:
ram_block_discard_range() cannot possibly do the right thing in
MAP_PRIVATE file mappings in the general case.
To achieve the documented semantics, we also have to punch a hole
into
the file, possibly messing with other MAP_PRIVATE/MAP_SHARED
mappings
of such a file.
For example, using VM templating -- see commit b17fbbe55cba
("migration:
allow private destination ram with x-ignore-shared") -- in
combination with
any mechanism that relies on discarding of RAM is problematic. This
includes:
* Postcopy live migration
* virtio-balloon inflation/deflation or free-page-reporting
* virtio-mem
So at least warn that there is something possibly dangerous is
going on
when using ram_block_discard_range() in these cases.
Acked-by: Peter Xu <pet...@redhat.com>
Tested-by: Mario Casquero <mcasq...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
softmmu/physmem.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/softmmu/physmem.c b/softmmu/physmem.c
index bda475a719..4ee157bda4 100644
--- a/softmmu/physmem.c
+++ b/softmmu/physmem.c
@@ -3456,6 +3456,24 @@ int ram_block_discard_range(RAMBlock *rb,
uint64_t start, size_t length)
* so a userfault will trigger.
*/
#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
+ /*
+ * We'll discard data from the actual file, even though
we only
+ * have a MAP_PRIVATE mapping, possibly messing with
other
+ * MAP_PRIVATE/MAP_SHARED mappings. There is no easy
way to
+ * change that behavior whithout violating the promised
+ * semantics of ram_block_discard_range().
+ *
+ * Only warn, because it work as long as nobody else
uses that
+ * file.
+ */
+ if (!qemu_ram_is_shared(rb)) {
+ warn_report_once("ram_block_discard_range:
Discarding RAM"
+ " in private file mappings is
possibly"
+ " dangerous, because it will
modify
the"
+ " underlying file and will affect
other"
+ " users of the file");
+ }
+
TDX has two types of memory backend for each RAM, shared memory and
private memory. Private memory is serviced by guest memfd and shared
memory can also be backed with a fd.
At any time, only one type needs to be valid, which means the
opposite
can be discarded. We do implement the memory discard when TDX
converts
the memory[1]. It will trigger this warning 100% because by
default the
guest memfd is not mapped as shared (MAP_SHARED).
If MAP_PRIVATE is not involved and you are taking the pages
directly out
of the memfd, you should mark that thing as shared.
Is it the general rule of Linux? Of just the rule of QEMU memory
discard?
MAP_SHARED vs. MAP_PRIVATE is a common UNIX principle, and that's what
this flag and the check is about.
From mmap(2)
MAP_SHARED: Share this mapping. Updates to the mapping are visible to
other processes mapping the same region, and (in the case of file-backed
mappings) are carried through to the underlying file.
MAP_PRIVATE: Create a private copy-on-write mapping. Updates to the
mapping are not visible to other processes mapping the same file, and
are not carried through to the underlying file. It is unspecified
whether changes made to the file after the mmap() call are visible in
the mapped region.
For your purpose (no mmap() at all), we behave like MAP_SHARED -- as if
nothing special is done. No Copy-on-write, no anonymous memory.
Anonymous memory is never involved.
Could you please elaborate more on this? What do you want to express
here regrading anonymous memory? (Sorry that I'm newbie for mmap stuff)
Anonymous memory is memory that is private to a specific process, and
(see MAP_PRIVATE) modifications remain private to the process and are
not reflected to the file.
If you have a MAP_PRIVATE file mapping and write to a virtual memory
location, you'll get a process-private copy of the underlying pagecache
page. that's what we call anonymous memory, because it does not belong
to a specific file. fallocate(punch) would not free up that anonymous
memory.
For guest memfd, it does implement kvm_gmem_fallocate as .fallocate()
callback, which calls truncate_inode_pages_range() [*].
I'm not sure if it frees up the memory. I need to learn it.
[*]
https://github.com/kvm-x86/linux/blob/911b515af3ec5f53992b9cc162cf7d3893c2fbe2/virt/kvm/guest_memfd.c#L147C73-L147C73
"Private memory" is only private from the guest POV, not from a mmap()
point of view.
Two different concepts of "private".
Simply remove the warning will fail the purpose of this patch. The
other
option is to skip the warning for TDX case, which looks vary
hacky. Do
you have any idea?
For TDX, all memory backends / RAMBlocks should be marked as "shared",
and you should fail if that is not provided by the user.
As I asked above, I want to understand the logic clearly. Is mapped as
shared is a must to support the memory discard? i.e., if we want to
support memory discard after memory type change, then the memory
must be
mapped with MAP_SHARED?
MAP_PIRVATE means that it's not sufficient to only fallocate(punch) the
fd to free up all memory for a virtual address, because there might be
anonymous memory in a private mapping that has to be freed up using
MADV_DONTNEED.
I can understand this. But it seems unrelated to my question: Is mapped
as shared is a must to support the memory discard?
Sorry, I don't quite get what you are asking that I haven't answered
yet. Let's talk about the issue you are seeing below.
e.g., if use below parameters to specify the RAM for a VM
-object memory-backend-memfd,id=mem0,size=2G \
-machine memory-backend=mem0
since not specifying "share" property, the ram_block doesn't have
RAM_SHARED set. If want to discard some range of this memfd, it triggers
the warning. Is this warning expected?
That should not be the case. See "memfd_backend_instance_init" where we
set share=true. In memfd_backend_memory_alloc(), we set RAM_SHARED.
We only default to share=off for memory-backend-file (well, and
memory-backend-ram).
So are you sure you get this error message in the configuration you are
describing here?
Sorry, I made an mistake. I was using "-object
memory-backend-ram,id=mem0,size=2G" instead of "memory-backend-memfd".
yes, when using "memory-backend-memfd" as the backend for TDX shared
memory, it doesn't trigger the warning because
memfd_backend_instance_init() set "share" to true.
When using "memory-backend-ram" as the backend for TDX shared memory,
the warning is triggered converting memory from private (kvm gmem) to
shared (backend-ram). In this case, there is a valid fd (kvm gmem fd),
so it goes to the path of need_fallocate. However,
qemu_ram_is_shared(rb) returns false because "memory-backend-ram"
doesn't have "share" default on. Thus, the warning is triggered.
It seems I need figure out a more proper solution to refactor the
ram_block_discard_range(), to make it applicable for kvm gmem discard case.
You should probably completely ignore any ramblock flags when
fallocate(punch) the kvm_gmem_fd. kvm_gmem_fd is a rather special
"secondary storage that's never mapped", independent of the ordinary
RAMBlock memory.
--
Cheers,
David / dhildenb
