Re: [PATCH kernel v8 3/4] mm: add inerface to offer info about unused pages

["Michael S. Tsirkin" <mst@redhat.com>]

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@intel.com> wrote:
> > 
> > > From: Liang Li <liang.z.li@intel.com>
> > > 
> > > 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

> > 
> > > +/*
> > > + * The record_unused_pages() function is used to record the system unused
> > > + * pages. The unused pages can be skipped to transfer during live migration.
> > > + * Though the unused pages are dynamically changing, dirty page logging
> > > + * mechanisms are able to capture the newly used pages though they were
> > > + * recorded as unused pages via this function.
> > > + *
> > > + * This function scans the free page list of the specified order to record
> > > + * the unused pages, and chunks those continuous pages following the chunk
> > > + * format below:
> > > + * --------------------------------------
> > > + * |	Base (52-bit)	| Rsvd (12-bit) |
> > > + * --------------------------------------
> > > + * --------------------------------------
> > > + * |	Size (52-bit)	| Rsvd (12-bit) |
> > > + * --------------------------------------
> > > + *
> > > + * @start_zone: zone to start the record operation.
> > > + * @order: order of the free page list to record.
> > > + * @buf: buffer to record the unused page info in chunks.
> > > + * @size: size of the buffer in __le64 to record
> > > + * @offset: offset in the buffer to record.
> > > + * @part_fill: indicate if partial fill is used.
> > > + *
> > > + * return -EINVAL if parameter is invalid
> > > + * return -ENOSPC when the buffer is too small to record all the unsed pages
> > > + * return 0 when sccess
> > > + */
> > It's a strange thing - it returns information which will instantly
> > become incorrect.
> 
> I didn't get the point, could you please explain more? Thanks.
> 
> Best,
> Wei

I tried to explain it in my reply.  Once this function drops the lock,
the pages can immediately be used so the returned value is wrong.
balloon uses tricks to recover from this but this needs to be explicit
at the API level.

-- 
MST