linux-kernel - Re: [PATCH v15 4/5] mm: support reporting free page blocks

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Date:   Mon, 28 Aug 2017 15:33:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v15 4/5] mm: support reporting free page blocks

On Mon 28-08-17 18:08:32, Wei Wang wrote:
> This patch adds support to walk through the free page blocks in the
> system and report them via a callback function. Some page blocks may
> leave the free list after zone->lock is released, so it is the caller's
> responsibility to either detect or prevent the use of such pages.
> 
> One use example of this patch is to accelerate live migration by skipping
> the transfer of free pages reported from the guest. A popular method used
> by the hypervisor to track which part of memory is written during live
> migration is to write-protect all the guest memory. So, those pages that
> are reported as free pages but are written after the report function
> returns will be captured by the hypervisor, and they will be added to the
> next round of memory transfer.

OK, looks much better. I still have few nits.

> +extern void walk_free_mem_block(void *opaque,
> +				int min_order,
> +				bool (*report_page_block)(void *, unsigned long,
> +							  unsigned long));
> +

please add names to arguments of the prototype

>  /*
>   * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
>   * into the buddy system. The freed pages will be poisoned with pattern
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6d00f74..81eedc7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
>  	show_swap_cache_info();
>  }
>  
> +/**
> + * walk_free_mem_block - Walk through the free page blocks in the system
> + * @opaque: the context passed from the caller
> + * @min_order: the minimum order of free lists to check
> + * @report_page_block: the callback function to report free page blocks

page_block has meaning in the core MM which doesn't strictly match its
usage here. Moreover we are reporting pfn ranges rather than struct page
range. So report_pfn_range would suit better.

[...]
> +	for_each_populated_zone(zone) {
> +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> +			for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) {
> +				spin_lock_irqsave(&zone->lock, flags);
> +				list = &zone->free_area[order].free_list[mt];
> +				list_for_each_entry(page, list, lru) {
> +					pfn = page_to_pfn(page);
> +					stop = report_page_block(opaque, pfn,
> +								 1 << order);
> +					if (stop)
> +						break;

					if (stop) {
						spin_unlock_irqrestore(&zone->lock, flags);
						return;
					}

would be both easier and less error prone. E.g. You wouldn't pointlessly
iterate over remaining orders just to realize there is nothing to be
done for those...

> +				}
> +				spin_unlock_irqrestore(&zone->lock, flags);
> +			}
> +		}
> +	}
> +}
> +EXPORT_SYMBOL_GPL(walk_free_mem_block);

-- 
Michal Hocko
SUSE Labs