Re: [PATCH v2 5/6] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination

[David Hildenbrand <david@redhat.com>]

On 29.07.21 21:20, Peter Xu wrote:
> On Thu, Jul 29, 2021 at 06:15:58PM +0200, David Hildenbrand wrote:
>> On 29.07.21 17:52, Peter Xu wrote:
>>> On Thu, Jul 29, 2021 at 02:14:41PM +0200, David Hildenbrand wrote:
>>>> On 24.07.21 00:10, Peter Xu wrote:
>>>>> On Fri, Jul 23, 2021 at 09:01:42PM +0200, David Hildenbrand wrote:
>>>>>> It can happen in corner cases and is valid: with the current virtio-mem
>>>>>> spec, guests are allowed to read unplugged memory. This will, for example,
>>>>>> happen on older Linux guests when reading /proc/kcore or (with even older
>>>>>> guests) when dumping guest memory via kdump. These corner cases were the
>>>>>> main reason why the spec allows for it -- until we have guests properly
>>>>>> adjusted such that it won't happen even in corner cases.
>>>>>>
>>>>>> A future feature bit will disallow it for the guest: required for supporting
>>>>>> shmem/hugetlb cleanly. With that in place, I agree that we would want to
>>>>>> warn in this case!
>>>>>
>>>>> OK that makes sense; with the page_size change, feel free to add:
>>>>
>>>> I just realized that relying on the page_size would be wrong.
>>>>
>>>> We migrate TARGET_PAGE_SIZE chunks and the offset might not be page_size
>>>> aligned. So if we were to replace TARGET_PAGE_SIZE by rb->page_size, we
>>>> might accidentally cover a "too big" range.
>>>
>>> I'm wondering whether we should make the offset page size aligned instead.  For
>>> example, note that postcopy_place_page_zero() should only take page_size
>>> aligned host addr or UFFDIO_COPY could fail (hugetlb doesn't support
>>> UFFDIO_ZEROPAGE yet).
>>
>> That is true indeed. I'd assume in that case that we would get called with
>> the proper offset already, right? Because uffd would only report properly
>> aligned pages IIRC.
> 
> Nop; it should return the faulted address. So postcopy_request_page() may need
> some alignment work, as it was handled in migrate_send_rp_req_pages().
> 

Right, figured that out myself just now:

static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb,
                                  ram_addr_t start, uint64_t haddr)
{
     void *aligned = (void *)(uintptr_t)(haddr & -qemu_ram_pagesize(rb));

     /*
      * Discarded pages (via RamDiscardManager) are never migrated. On unlikely
      * access, place a zeropage, which will also set the relevant bits in the
      * recv_bitmap accordingly, so we won't try placing a zeropage twice.
      *
      * Checking a single bit is sufficient to handle pagesize > TPS as either
      * all relevant bits are set or not.
      */
     assert(QEMU_IS_ALIGNED(start, qemu_ram_pagesize(rb)));
     if (ramblock_page_is_discarded(rb, start)) {
         bool received = ramblock_recv_bitmap_test_byte_offset(rb, start);

         return received ? 0 : postcopy_place_page_zero(mis, aligned, rb);
     }

     return migrate_send_rp_req_pages(mis, rb, start, haddr);
}


-- 
Thanks,

David / dhildenb