Re: [PATCH kernel v8 3/4] mm: add inerface to offer info about unused pages
From: Wei Wang
Date: Fri Mar 31 2017 - 05:51:35 EST
On 03/30/2017 01:48 AM, Michael S. Tsirkin wrote:
On Fri, Mar 17, 2017 at 02:55:33PM +0800, Wei Wang wrote:
On 03/17/2017 05:28 AM, Andrew Morton wrote:
On Thu, 16 Mar 2017 15:08:46 +0800 Wei Wang <wei.w.wang@xxxxxxxxx> wrote:
From: Liang Li <liang.z.li@xxxxxxxxx>
This patch adds a function to provides a snapshot of the present system
unused pages. An important usage of this function is to provide the
unsused pages to the Live migration thread, which skips the transfer of
thoses unused pages. Newly used pages can be re-tracked by the dirty
page logging mechanisms.
I don't think this will be useful for anything other than
virtio-balloon. I guess it would be better to keep this code in the
virtio-balloon driver if possible, even though that's rather a layering
violation :( What would have to be done to make that possible? Perhaps
we can put some *small* helpers into page_alloc.c to prevent things
from becoming too ugly.
The patch description was too narrowed and may have caused some
confusion, sorry about that. This function is aimed to be generic. I
agree with the description suggested by Michael.
Since the main body of the function is related to operating on the
free_list. I think it is better to have them located here.
Small helpers may be less efficient and thereby causing some
performance loss as well.
I think one improvement we can make is to remove the "chunk format"
related things from this function. The function can generally offer the
base pfn to the caller's recording buffer. Then it will be the caller's
responsibility to format the pfn if they need.
Sounds good at a high level, but we'd have to see the implementation
to judge it properly.
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4498,6 +4498,120 @@ void show_free_areas(unsigned int filter)
show_swap_cache_info();
}
+static int __record_unused_pages(struct zone *zone, int order,
+ __le64 *buf, unsigned int size,
+ unsigned int *offset, bool part_fill)
+{
+ unsigned long pfn, flags;
+ int t, ret = 0;
+ struct list_head *curr;
+ __le64 *chunk;
+
+ if (zone_is_empty(zone))
+ return 0;
+
+ spin_lock_irqsave(&zone->lock, flags);
+
+ if (*offset + zone->free_area[order].nr_free > size && !part_fill) {
+ ret = -ENOSPC;
+ goto out;
+ }
+ for (t = 0; t < MIGRATE_TYPES; t++) {
+ list_for_each(curr, &zone->free_area[order].free_list[t]) {
+ pfn = page_to_pfn(list_entry(curr, struct page, lru));
+ chunk = buf + *offset;
+ if (*offset + 2 > size) {
+ ret = -ENOSPC;
+ goto out;
+ }
+ /* Align to the chunk format used in virtio-balloon */
+ *chunk = cpu_to_le64(pfn << 12);
+ *(chunk + 1) = cpu_to_le64((1 << order) << 12);
+ *offset += 2;
+ }
+ }
+
+out:
+ spin_unlock_irqrestore(&zone->lock, flags);
+
+ return ret;
+}
This looks like it could disable interrupts for a long time. Too long?
What do you think if we give "budgets" to the above function?
For example, budget=1000, and there are 2000 nodes on the list.
record() returns with "incomplete" status in the first round, along with the
status info, "*continue_node".
*continue_node: pointer to the starting node of the leftover. If
*continue_node
has been used at the time of the second call (i.e. continue_node->next ==
NULL),
which implies that the previous 1000 nodes have been used, then the record()
function can simply start from the head of the list.
It is up to the caller whether it needs to continue the second round
when getting "incomplete".
It might be cleaner to add APIs to
- start iteration
- do one step
- end iteration
caller can then iterate without too many issues
OK. I will re-implement it with this simple one - get only one node(page
block) from the list in each call, and check if the time would increase
a lot in comparison to v8.
Best,
Wei