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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.02.14 at 12:03, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> On 02/19/2014 08:55 AM, Jan Beulich wrote:
>>>>> On 19.02.14 at 02:28, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> 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".

Jan


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