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SubjectRe: [Xen-devel] [PATCH] drivers: xen-selfballoon: consider slab pages

On 11/05/2013 01:22 AM, David Vrabel wrote:
> On 04/11/13 12:39, Bob Liu wrote:
>> Currently the goal_page in xen-selfballon doesn't consider much about pages used
>> in kernel space.
>> A typical usage is slab pages, without consider slab pages the goal_page result
>> may be too rough and lead extra memory pressure to guest os.
>
> Can you provide some real world figures where the calculatation got it
> wrong? What was the resultant behavior? Swap death? OOM killer?
>

Sorry, I didn't run any testing I just think it's unreasonable while
reading the source code.

vm_memory_committed() only calculate pages which mapped to process
address space, but the kernel itself(like block, fs and network
subsystem) may occupy some memory. And it's possible that those
subsystem may occupy a significant amount of memory in some situation.

I'm afraid if we don't consider those kernel memory while calculating
goal_pages, guest memory will be set lower than guest really needs.

>> Signed-off-by: Bob Liu <bob.liu@oracle.com>
>> ---
>> drivers/xen/xen-selfballoon.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
>> index 21e18c1..4814759 100644
>> --- a/drivers/xen/xen-selfballoon.c
>> +++ b/drivers/xen/xen-selfballoon.c
>> @@ -191,6 +191,8 @@ static void selfballoon_process(struct work_struct *work)
>> tgt_pages = cur_pages; /* default is no change */
>> goal_pages = vm_memory_committed() +
>> totalreserve_pages +
>> + global_page_state(NR_SLAB_RECLAIMABLE) +
>
> Does SLAB_RECLAIMABLE want to be included here? Unless I'm
> misunderstanding here, SLAB_RECLAIMABLE is effectively free.
>

SLAB_RECLAIMABLE isn't effectively free, it means the slab page is in
used but can be reclaimed(freed) during memory pressure.

>> + global_page_state(NR_SLAB_UNRECLAIMABLE) +
>
> This bit looks fine to me.
>
>> MB2PAGES(selfballoon_reserved_mb);
>> #ifdef CONFIG_FRONTSWAP
>> /* allow space for frontswap pages to be repatriated */
>
> David
>

Thanks for your review.

--
Regards,
-Bob


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