Re: [PATCH V2] memory: RAM_NAMED_FILE flag

[Steven Sistare]

On 2/8/2023 2:22 PM, Peter Xu wrote:
> On Wed, Feb 08, 2023 at 01:34:18PM -0500, Steven Sistare wrote:
>> On 2/7/2023 3:23 PM, Peter Xu wrote:
>>> On Tue, Feb 07, 2023 at 11:03:33AM -0800, Steve Sistare wrote:
>>>> migrate_ignore_shared() is an optimization that avoids copying memory
>>>> that is visible and can be mapped on the target.  However, a
>>>> memory-backend-ram or a memory-backend-memfd block with the RAM_SHARED
>>>> flag set is not migrated when migrate_ignore_shared() is true.  This is
>>>> wrong, because the block has no named backing store, and its contents will
>>>> be lost.  To fix, ignore shared memory iff it is a named file.  Define a
>>>> new flag RAM_NAMED_FILE to distinguish this case.
>>>
>>> There's also TYPE_MEMORY_BACKEND_EPC.  Reading the commit message it seems
>>> it can still be used in similar ways.  Pasting commit message from c6c0232:
>>>
>>>     Because of its unique requirements, Linux manages EPC separately from
>>>     normal memory.  Similar to memfd, the device /dev/sgx_vepc can be
>>>     opened to obtain a file descriptor which can in turn be used to mmap()
>>>     EPC memory.
>>>
>>> I'm not sure whether it means that should apply for RAM_NAMED_FILE too,
>>> neither do I think it's super important..  Still better to define it
>>> properly.
>>
>> The RAM_NAMED_FILE flag will be false for TYPE_MEMORY_BACKEND_EPC, so 
>> ramblock_is_ignored will return false, and the contents will be copied
>> over the socket to the target, and the segment will be recreated there.
>>
>> However, perhaps I do not understand your point.
> 
> My point was it looked like it should apply RAM_NAMED_FILE too, because
> it's also a named file.
> 
> But..  I don't think another QEMU can share the same data if opening the
> same file.  Based on my zero knowledge on it... I quickly looked up
> sgx_vepc_open() in the Linux impl where sgx_vepc.page_array plays a role of
> page cache iiuc, while it's private per vma even if VM_SHARED.
> 
> So please ignore my comment..
> 
>>
>>> Another comment is, AFAIK this patch will modify senamtics of the old
>>> capability "x-ignore-shared".  But I'd say in a sensible way.  Maybe worth
>>> directly modify qapi/migration.json to reflect it (especially it's x-
>>> prefixed) to something like:
>>>
>>> # @x-ignore-shared: If enabled, QEMU will not migrate named shared memory
>>> #                   (since 4.0) 
>>
>> Good idea.  I propose:
>>
>> # @x-ignore-shared: If enabled, QEMU will not migrate shared memory that is
>> #                   accessible on the target. (since 4.0)
> 
> I would use s/target/destination host/ because target can be misread as the
> VM itself (at least a general term in migration code base).  Other than
> that looks good to me.

Hi Peter, I will rebase to the tip, modify the wording, and repost.  
Can I add your RB?

- Steve