Re: [PATCH v3 7/7] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 8/5/21 10:11 AM, David Hildenbrand wrote:
> On 05.08.21 10:04, Philippe Mathieu-Daudé wrote:
>> On 7/30/21 10:52 AM, David Hildenbrand wrote:
>>> We already don't ever migrate memory that corresponds to discarded
>>> ranges
>>> as managed by a RamDiscardManager responsible for the mapped memory
>>> region
>>> of the RAMBlock.
>>>
>>> virtio-mem uses this mechanism to logically unplug parts of a RAMBlock.
>>> Right now, we still populate zeropages for the whole usable part of the
>>> RAMBlock, which is undesired because:
>>>
>>> 1. Even populating the shared zeropage will result in memory getting
>>>     consumed for page tables.
>>> 2. Memory backends without a shared zeropage (like hugetlbfs and shmem)
>>>     will populate an actual, fresh page, resulting in an unintended
>>>     memory consumption.
>>>
>>> Discarded ("logically unplugged") parts have to remain discarded. As
>>> these pages are never part of the migration stream, there is no need to
>>> track modifications via userfaultfd WP reliably for these parts.
>>>
>>> Further, any writes to these ranges by the VM are invalid and the
>>> behavior is undefined.
>>>
>>> Note that Linux only supports userfaultfd WP on private anonymous memory
>>> for now, which usually results in the shared zeropage getting populated.
>>> The issue will become more relevant once userfaultfd WP supports shmem
>>> and hugetlb.
>>>
>>> Acked-by: Peter Xu <peterx@redhat.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>   migration/ram.c | 53 +++++++++++++++++++++++++++++++++++++++++--------
>>>   1 file changed, 45 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/migration/ram.c b/migration/ram.c
>>> index 01cea01774..fd5949734e 100644
>>> --- a/migration/ram.c
>>> +++ b/migration/ram.c
>>> @@ -1639,6 +1639,28 @@ out:
>>>       return ret;
>>>   }
>>>   +static inline void populate_range(RAMBlock *block, hwaddr offset,
>>> hwaddr size)
>>> +{
>>> +    char *ptr = (char *) block->host;
>>> +
>>> +    for (; offset < size; offset += qemu_real_host_page_size) {
>>> +        char tmp = *(ptr + offset);
>>> +
>>> +        /* Don't optimize the read out */
>>> +        asm volatile("" : "+r" (tmp));
>>> +    }
>>
>> This template is now used 3 times, a good opportunity to extract it as
>> an (inline?) helper.
>>
> 
> Can you point me at the other users?

Oops I got lost reviewing the series.

> Isn't populate_range() the inline helper you are looking for? :)

Indeed :)

Being a bit picky, I'd appreciate if you split this patch in 2,
extracting populate_range() as a first trivial step.