Re: [PATCH] kvm: Take into account the unaligned section size when preparing bitmap

[zhukeqian]

Hi,

I see that if start or size is not PAGE aligned, it also clears areas
which beyond caller's expectation, so do we also need to consider this?

Thanks,
Keqian

On 2020/12/9 10:33, Zenghui Yu wrote:
> Hi Peter,
> 
> Thanks for having a look at it.
> 
> On 2020/12/8 23:16, Peter Xu wrote:
>> Hi, Zenghui,
>>
>> On Tue, Dec 08, 2020 at 07:40:13PM +0800, Zenghui Yu wrote:
>>> The kernel KVM_CLEAR_DIRTY_LOG interface has align requirement on both the
>>> start and the size of the given range of pages. We have been careful to
>>> handle the unaligned cases when performing CLEAR on one slot. But it seems
>>> that we forget to take the unaligned *size* case into account when
>>> preparing bitmap for the interface, and we may end up clearing dirty status
>>> for pages outside of [start, start + size).
>>
>> Thanks for the patch, though my understanding is that this is not a bug.
>>
>> Please have a look at kvm_memslot_init_dirty_bitmap() where we'll allocate 
>> the
>> dirty bitmap to be aligned to 8 bytes (assuming that's the possible max of 
>> the
>> value sizeof(unsigned long)).  That exactly covers 64 pages.
>>
>> So here as long as start_delta==0 (so the value of "bmap_npages - size / 
>> psize"
>> won't really matter a lot, imho), then we'll definitely have 
>> KVMSlot.dirty_bmap
>> long enough to cover the range we'd like to clear.
> 
> I agree.  But actually I'm not saying that KVMSlot.dirty_bmap is not
> long enough.  What I was having in mind is something like:
> 
>     // psize = qemu_real_host_page_size;
>     // slot.start_addr = 0;
>     // slot.memory_size = 64 * psize;
> 
>     kvm_log_clear_one_slot(slot, as, 0 * psize, 32 * psize);   --> [1]
>     kvm_log_clear_one_slot(slot, as, 32 * psize, 32 * psize);  --> [2]
> 
> So the @size is not aligned with 64 pages.  Before this patch, we'll
> clear dirty status for all pages(0-63) through [1].  It looks to me that
> this violates the caller's expectation since we only want to clear
> pages(0-31).
> 
> As I said, I don't think this will happen in practice -- the migration
> code should always provide us with a 64-page aligned section (right?).
> I'm just thinking about the correctness of the specific algorithm used
> by kvm_log_clear_one_slot().
> 
> Or maybe I had missed some other points obvious ;-) ?
> 
> 
> Thanks,
> Zenghui
> 
>> Note that the size of KVMSlot.dirty_bmap can be bigger than the actually size
>> of the kvm memslot, however since kvm_memslot_init_dirty_bitmap() initialized
>> it to all zero so the extra bits will always be zero for the whole lifecycle 
>> of
>> the vm/bitmap.
>>
>> Thanks,
>>
>>>
>>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>>> ---
>>>   accel/kvm/kvm-all.c | 7 +++++--
>>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>>> index bed2455ca5..05d323ba1f 100644
>>> --- a/accel/kvm/kvm-all.c
>>> +++ b/accel/kvm/kvm-all.c
>>> @@ -747,7 +747,7 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int 
>>> as_id, uint64_t start,
>>>       assert(bmap_start % BITS_PER_LONG == 0);
>>>       /* We should never do log_clear before log_sync */
>>>       assert(mem->dirty_bmap);
>>> -    if (start_delta) {
>>> +    if (start_delta || bmap_npages - size / psize) {
>>>           /* Slow path - we need to manipulate a temp bitmap */
>>>           bmap_clear = bitmap_new(bmap_npages);
>>>           bitmap_copy_with_src_offset(bmap_clear, mem->dirty_bmap,
>>> @@ -760,7 +760,10 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int 
>>> as_id, uint64_t start,
>>>           bitmap_clear(bmap_clear, 0, start_delta);
>>>           d.dirty_bitmap = bmap_clear;
>>>       } else {
>>> -        /* Fast path - start address aligns well with BITS_PER_LONG */
>>> +        /*
>>> +         * Fast path - both start and size align well with BITS_PER_LONG
>>> +         * (or the end of memory slot)
>>> +         */
>>>           d.dirty_bitmap = mem->dirty_bmap + BIT_WORD(bmap_start);
>>>       }
>>>   -- 
>>> 2.19.1
>>>
>>>
>>
> 
> .
>