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Re: [Xen-devel] [PATCH v2 3/3] xen: use idle vcpus to scrub pages

On 07/25/2014 02:51 PM, Jan Beulich wrote:
>>>> On 25.07.14 at 02:42, <bob.liu@xxxxxxxxxx> wrote:
>> On 07/24/2014 02:24 PM, Jan Beulich wrote:
>>>>>> On 24.07.14 at 04:08, <bob.liu@xxxxxxxxxx> wrote:
>>>> On 07/23/2014 03:28 PM, Jan Beulich wrote:
>>>>>>>> On 15.07.14 at 11:16, <bob.liu@xxxxxxxxxx> wrote:
>>>>>> After so many days I haven't make a workable solution if don't remove
>>>>>> pages temporarily. The hardest part is iterating the heap free list
>>>>>> without holding heap_lock because if holding the lock it might be heavy
>>>>>> lock contention.
>>>>>> So do you think it's acceptable if fixed all other concerns about this
>>>>>> patch?
>>>>> No, I don't think so. Instead I'm of the opinion that you may have
>>>>> worked in the wrong direction: Rather than not taking the heap lock
>>>>> at all, it may also be sufficient to shrink the lock holding time (i.e.
>>>>> avoid long loops with the lock held).
>>>> But I still think have to drop pages from heap list temporarily else
>>>> heap lock must be taken for a long time to get rid of E.g. below race
>>>> condition.
>>>> A: alloc path            B: idle loop
>>>>                         spin_lock(&heap_lock)
>>>>                       page_list_for_each( pg, &heap(node, zone, order) )
>>>>                         if _PGC_need_scrub is set, break;
>>>>                         spin_unlock(&heap_lock)
>>>>                         if ( test_bit(_PGC_need_scrub, pg)
>>>> ^^^^
>>>> spin_lock(&heap_lock)
>>>> delist page
>>>> spin_unlock(&heap_lock)
>>>> write data to this page
>>>>                          scrub_one_page(pg)
>>>>                          ^^^ will clean useful data
>>> No (and I'm sure I said so before): The only problem is with the
>>> linked list itself; the page contents are not a problem - the
>>> allocation path can simply wait for the already suggested
>>> _PGC_scrubbing flag to clear before returning. And as already
>> The page contents are a problem if the race condition I mentioned in
>> previous email happen.
>> Because there is a time window between checking the PGC_need_scrub flag
>> and doing the real scrub in idle thread, the idle thread will still
>> scrub a page after that page have been allocated by allocation path and
>> been used(and have been written some useful data).
> Did you really read all of my previous reply?

Sure, may be I misunderstood your reply.
If the allocation path can wait for the flag there is no problem, but I
remember you suggested to do the scrubbing also in allocation path in
which case I think this race condition will happen.


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