Re: [PATCH v15 4/7] mm: Introduce Reported pages

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 12/5/19 11:22 AM, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>
> In order to pave the way for free page reporting in virtualized
> environments we will need a way to get pages out of the free lists and
> identify those pages after they have been returned. To accomplish this,
> this patch adds the concept of a Reported Buddy, which is essentially
> meant to just be the Uptodate flag used in conjunction with the Buddy
> page type.
>
> To prevent the reported pages from leaking outside of the buddy lists I
> added a check to clear the PageReported bit in the del_page_from_free_list
> function. As a result any reported page that is split, merged, or
> allocated will have the flag cleared prior to the PageBuddy value being
> cleared.
>
> The process for reporting pages is fairly simple. Once we free a page that
> meets the minimum order for page reporting we will schedule a worker thread
> to start 2s or more in the future. That worker thread will begin working
> from the lowest supported page reporting order up to MAX_ORDER - 1 pulling
> unreported pages from the free list and storing them in the scatterlist.
>
> When processing each individual free list it is necessary for the worker
> thread to release the zone lock when it needs to stop and report the full
> scatterlist of pages. To reduce the work of the next iteration the worker
> thread will rotate the free list so that the first unreported page in the
> free list becomes the first entry in the list.

[...]

> k);
> +
> +	return err;
> +}
> +
> +static int
> +page_reporting_process_zone(struct page_reporting_dev_info *prdev,
> +			    struct scatterlist *sgl, struct zone *zone)
> +{
> +	unsigned int order, mt, leftover, offset = PAGE_REPORTING_CAPACITY;
> +	unsigned long watermark;
> +	int err = 0;
> +
> +	/* Generate minimum watermark to be able to guarantee progress */
> +	watermark = low_wmark_pages(zone) +
> +		    (PAGE_REPORTING_CAPACITY << PAGE_REPORTING_MIN_ORDER);
> +
> +	/*
> +	 * Cancel request if insufficient free memory or if we failed
> +	 * to allocate page reporting statistics for the zone.
> +	 */
> +	if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
> +		return err;
> +


Will it not make more sense to check the low watermark condition before every
reporting request generated for a bunch of 32 isolated pages?
or will that be too costly?

> +	/* Process each free list starting from lowest order/mt */
> +	for (order = PAGE_REPORTING_MIN_ORDER; order < MAX_ORDER; order++) {
> +		for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> +			/* We do not pull pages from the isolate free list */
> +			if (is_migrate_isolate(mt))
> +				continue;
> +
> +			err = page_reporting_cycle(prdev, zone, order, mt,
> +						   sgl, &offset);
> +			if (err)
> +				return err;
> +		}
> +	}
> +
> +	/* report the leftover pages before going idle */
> +	leftover = PAGE_REPORTING_CAPACITY - offset;
> +	if (leftover) {
> +		sgl = &sgl[offset];
> +		err = prdev->report(prdev, sgl, leftover);
> +
> +		/* flush any remaining pages out from the last report */
> +		spin_lock_irq(&zone->lock);
> +		page_reporting_drain(prdev, sgl, leftover, !err);
> +		spin_unlock_irq(&zone->lock);
> +	}
> +
> +	return err;
> +}
-- 
Nitesh