Re: [Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

[Juan Quintela]

"Dr. David Alan Gilbert (git)"  wrote:
> From: "Dr. David Alan Gilbert" 
>
> My fix (84e7b80a) replaced the last_sent_block update that I'd
> removed earlier; however it was too aggressive in the xbzrle case.
>
> save_xbzrle_page might return '0' to mean that the page didn't
> need sending since it was the same as the last sent version;
> in this case we can't update 'last_sent_block' since we didn't
> actually send it.
>
> Symptom: 'Illegal RAM offset 1018000' as we try and send a page
> to the wrong RAMBlock;  potentially that could be a data
> corruption if you were really unlucky.
>
> Fixes: 84e7b80a05c0c44b90533c6cd2f1db5c932ccf77
>
> Signed-off-by: Dr. David Alan Gilbert 

Reviewed-by: Juan Quintela 

Re: [Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

[Juan Quintela]

Peter Maydell  wrote:
D> On 10 December 2015 at 16:31, Dr. David Alan Gilbert (git)
>  wrote:
>> From: "Dr. David Alan Gilbert" 
>>
>> My fix (84e7b80a) replaced the last_sent_block update that I'd
>> removed earlier; however it was too aggressive in the xbzrle case.
>>
>> save_xbzrle_page might return '0' to mean that the page didn't
>> need sending since it was the same as the last sent version;
>> in this case we can't update 'last_sent_block' since we didn't
>> actually send it.
>>
>> Symptom: 'Illegal RAM offset 1018000' as we try and send a page
>> to the wrong RAMBlock;  potentially that could be a data
>> corruption if you were really unlucky.
>>
>> Fixes: 84e7b80a05c0c44b90533c6cd2f1db5c932ccf77
>>
>> Signed-off-by: Dr. David Alan Gilbert 
>> ---
>>  migration/ram.c | 11 ++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 1eb155a..0490f00 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -716,6 +716,9 @@ static int save_zero_page(QEMUFile *f, RAMBlock *block, 
>> ram_addr_t offset,
>>   * ram_save_page: Send the given page to the stream
>>   *
>>   * Returns: Number of pages written.
>> + *  < 0 - error
>> + *  >=0 - Number of pages written - this might legally be 0
>> + *if xbzrle noticed the page was the same.
>>   *
>>   * @f: QEMUFile where to send the data
>>   * @block: block that contains the page we want to send
>> @@ -1249,7 +1252,13 @@ static int ram_save_target_page(MigrationState *ms, 
>> QEMUFile *f,
>>  if (unsentmap) {
>>  clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
>>  }
>> -last_sent_block = block;
>> +/* Only update last_sent_block if a block was actually sent; xbzrle
>> + * might have decided the page was identical so didn't bother 
>> writing
>> + * to the stream.
>> + */
>> +if (res > 0) {
>> +last_sent_block = block;
>> +}
>>  }
>>
>>  return res;
>
> This sounds like we should probably put this into 2.5; I'm happy
> to do so if it gets review by tomorrow afternoon and Juan/Amit
> agree.

Yeap, did the review by.  Do you want a pull request, or just pick it
directly?

Re: [Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

[Peter Maydell]

On 11 December 2015 at 12:52, Peter Maydell  wrote:
> On 11 December 2015 at 12:25, Juan Quintela  wrote:
>> Peter Maydell  wrote:
>>> This sounds like we should probably put this into 2.5; I'm happy
>>> to do so if it gets review by tomorrow afternoon and Juan/Amit
>>> agree.
>>
>> Yeap, did the review by.  Do you want a pull request, or just pick it
>> directly?
>
> I'll just apply it directly.

Now applied, thanks.

-- PMM

Re: [Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

[Peter Maydell]

On 11 December 2015 at 12:25, Juan Quintela  wrote:
> Peter Maydell  wrote:
>> This sounds like we should probably put this into 2.5; I'm happy
>> to do so if it gets review by tomorrow afternoon and Juan/Amit
>> agree.
>
> Yeap, did the review by.  Do you want a pull request, or just pick it
> directly?

I'll just apply it directly.

thanks
-- PMM

Re: [Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

[Peter Maydell]

On 10 December 2015 at 16:31, Dr. David Alan Gilbert (git)
 wrote:
> From: "Dr. David Alan Gilbert" 
>
> My fix (84e7b80a) replaced the last_sent_block update that I'd
> removed earlier; however it was too aggressive in the xbzrle case.
>
> save_xbzrle_page might return '0' to mean that the page didn't
> need sending since it was the same as the last sent version;
> in this case we can't update 'last_sent_block' since we didn't
> actually send it.
>
> Symptom: 'Illegal RAM offset 1018000' as we try and send a page
> to the wrong RAMBlock;  potentially that could be a data
> corruption if you were really unlucky.
>
> Fixes: 84e7b80a05c0c44b90533c6cd2f1db5c932ccf77
>
> Signed-off-by: Dr. David Alan Gilbert 
> ---
>  migration/ram.c | 11 ++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 1eb155a..0490f00 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -716,6 +716,9 @@ static int save_zero_page(QEMUFile *f, RAMBlock *block, 
> ram_addr_t offset,
>   * ram_save_page: Send the given page to the stream
>   *
>   * Returns: Number of pages written.
> + *  < 0 - error
> + *  >=0 - Number of pages written - this might legally be 0
> + *if xbzrle noticed the page was the same.
>   *
>   * @f: QEMUFile where to send the data
>   * @block: block that contains the page we want to send
> @@ -1249,7 +1252,13 @@ static int ram_save_target_page(MigrationState *ms, 
> QEMUFile *f,
>  if (unsentmap) {
>  clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
>  }
> -last_sent_block = block;
> +/* Only update last_sent_block if a block was actually sent; xbzrle
> + * might have decided the page was identical so didn't bother writing
> + * to the stream.
> + */
> +if (res > 0) {
> +last_sent_block = block;
> +}
>  }
>
>  return res;

This sounds like we should probably put this into 2.5; I'm happy
to do so if it gets review by tomorrow afternoon and Juan/Amit
agree.

thanks
-- PMM