[RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[Nitesh Narayan Lal]

The following patch-set proposes an efficient mechanism for handing freed memory between the guest and the host. It enables the guests with no page cache to rapidly free and reclaims memory to and from the host respectively.

Benefit:
With this patch-series, in our test-case, executed on a single system and single NUMA node with 15GB memory, we were able to successfully launch atleast 5 guests 
when page hinting was enabled and 3 without it. (Detailed explanation of the test procedure is provided at the bottom).

Changelog in V8:
In this patch-series, the earlier approach [1] which was used to capture and scan the pages freed by the guest has been changed. The new approach is briefly described below:

The patch-set still leverages the existing arch_free_page() to add this functionality. It maintains a per CPU array which is used to store the pages freed by the guest. The maximum number of entries which it can hold is defined by MAX_FGPT_ENTRIES(1000). When the array is completely filled, it is scanned and only the pages which are available in the buddy are stored. This process continues until the array is filled with pages which are part of the buddy free list. After which it wakes up a kernel per-cpu-thread.
This kernel per-cpu-thread rescans the per-cpu-array for any re-allocation and if the page is not reallocated and present in the buddy, the kernel thread attempts to isolate it from the buddy. If it is successfully isolated, the page is added to another per-cpu array. Once the entire scanning process is complete, all the isolated pages are reported to the host through an existing virtio-balloon driver.

Known Issues:
	* Fixed array size: The problem with having a fixed/hardcoded array size arises when the size of the guest varies. For example when the guest size increases and it starts making large allocations fixed size limits this solution's ability to capture all the freed pages. This will result in less guest free memory getting reported to the host.

Known code re-work:
	* Plan to re-use Wei's work, which communicates the poison value to the host.
	* The nomenclatures used in virtio-balloon needs to be changed so that the code can easily be distinguished from Wei's Free Page Hint code.
	* Sorting based on zonenum, to avoid repetitive zone locks for the same zone.

Other required work:
	* Run other benchmarks to evaluate the performance/impact of this approach.

Test case:
Setup:
Memory-15837 MB
Guest Memory Size-5 GB
Swap-Disabled
Test Program-Simple program which allocates 4GB memory via malloc, touches it via memset and exits.
Use case-Number of guests that can be launched completely including the successful execution of the test program.
Procedure: 
The first guest is launched and once its console is up, the test allocation program is executed with 4 GB memory request (Due to this the guest occupies almost 4-5 GB of memory in the host in a system without page hinting). Once this program exits at that time another guest is launched in the host and the same process is followed. We continue launching the guests until a guest gets killed due to low memory condition in the host.

Result:
Without Hinting-3 Guests
With Hinting-5 to 7 Guests(Based on the amount of memory freed/captured).

[1] https://www.spinics.net/lists/kvm/msg170113.html