Re: [PATCH v2 1/4] softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping

[David Hildenbrand <david@redhat.com>]

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 <peterx@redhat.com>
>> Tested-by: Mario Casquero <mcasquer@redhat.com>
>> Signed-off-by: David Hildenbrand <david@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. Anonymous memory is 
never involved.

"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.

-- 
Cheers,

David / dhildenb