Re: [PATCH] Don't track all memory when enabling log dirty to track vram

[George Dunlap <george.dunlap@eu.citrix.com>]

On 02/19/2014 11:13 AM, Jan Beulich wrote:
>>>> On 19.02.14 at 12:03, George Dunlap <george.dunlap@eu.citrix.com> wrote:
>> On 02/19/2014 08:55 AM, Jan Beulich wrote:
>>>>>> On 19.02.14 at 02:28, "Zhang, Yang Z" <yang.z.zhang@intel.com> wrote:
>>>> George Dunlap wrote on 2014-02-18:
>>>>> On 02/18/2014 03:14 AM, Zhang, Yang Z wrote:
>>>>> perhaps my original patch is better which will check
>>>>> paging_mode_log_dirty(d) && log_global:
>>>>>
>>>>> It turns out that the reason I couldn't get a crash was because libxc
>>>>> was actually paying attention to the -EINVAL return value, and
>>>>> disabling and then re-enabling logdirty.  That's what would happen
>>>>> before your dirty vram patch, and that's what happens after.  And
>>>>> arguably, that's the correct behavior for any toolstack, given that the
>>>> interface returns an error.
>>>>
>>>> Agree.
>>>>
>>>>> This patch would actually change the interface; if we check this in,
>>>>> then if you enable logdirty when dirty vram tracking is enabled, you
>>>>> *won't* get an error, and thus *won't* disable and re-enable logdirty mode.
>>>>> So actually, this patch would be more disruptive.
>>>>>
>>>> Jan, do you have any comment?
>>> This simplistic variant is just calling for problems. As was already
>>> said elsewhere on this thread, we should simply do the mode change
>>> properly: Track that a partial log-dirty mode is in use, and allow
>>> switching to global log-dirty mode (converting all entries to R/O).
>> I think Yang was asking you for your opinion on my suggestion that
>> nothing actually needed to be done.  Enabling full logdirty mode for
>> migration when dirty vram tracking was enabled has *always* returned an
>> error (or at least for a long time now), and *always* resulted in the
>> toolstack disabling and re-enabling logdirty mode; Yang's patch doesn't
>> change that at all.
>>
>> If you think that's an interface we need to improve in the future, we
>> can put it on the list of improvements.  But at this point it seems to
>> me more like a nice-to-have.
> I agree - for 4.4.0 we shouldn't need any further adjustments. And
> I hoped to imply that I don't see a need for this incremental change
> to go in by having said "This simplistic variant is just calling for
> problems".

No, but "we should simply do the mode change properly" could be 
interpreted as saying, "this needs to be done as a follow-up to the 
dirty vram tracking patch"; someone might even interpret it as, "you 
need to do this as a follow-up".  That's what I was trying to clarify / 
express an opinion on. :-)

  -George