linux-kernel - Re: [PATCH v33 1/4] mm: add a function to get free page blocks

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Date:   Fri, 15 Jun 2018 21:50:05 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mst@...hat.com, mhocko@...nel.org,
        akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com, peterx@...hat.com
Subject: Re: [PATCH v33 1/4] mm: add a function to get free page blocks

On Fri, Jun 15, 2018 at 12:43:10PM +0800, Wei Wang wrote:
> +/**
> + * get_from_free_page_list - get free page blocks from a free page list
> + * @order: the order of the free page list to check
> + * @buf: the array to store the physical addresses of the free page blocks
> + * @size: the array size
> + *
> + * This function offers hints about free pages. There is no guarantee that
> + * the obtained free pages are still on the free page list after the function
> + * returns. pfn_to_page on the obtained free pages is strongly discouraged
> + * and if there is an absolute need for that, make sure to contact MM people
> + * to discuss potential problems.
> + *
> + * The addresses are currently stored to the array in little endian. This
> + * avoids the overhead of converting endianness by the caller who needs data
> + * in the little endian format. Big endian support can be added on demand in
> + * the future.
> + *
> + * Return the number of free page blocks obtained from the free page list.
> + * The maximum number of free page blocks that can be obtained is limited to
> + * the caller's array size.
> + */

Please use:

 * Return: The number of free page blocks obtained from the free page list.

Also, please include a

 * Context: Any context.

or

 * Context: Process context.

or whatever other conetext this function can be called from.  Since you're
taking the lock irqsafe, I assume this can be called from any context, but
I wonder if it makes sense to have this function callable from interrupt
context.  Maybe this should be callable from process context only.

> +uint32_t get_from_free_page_list(int order, __le64 buf[], uint32_t size)
> +{
> +	struct zone *zone;
> +	enum migratetype mt;
> +	struct page *page;
> +	struct list_head *list;
> +	unsigned long addr, flags;
> +	uint32_t index = 0;
> +
> +	for_each_populated_zone(zone) {
> +		spin_lock_irqsave(&zone->lock, flags);
> +		for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> +			list = &zone->free_area[order].free_list[mt];
> +			list_for_each_entry(page, list, lru) {
> +				addr = page_to_pfn(page) << PAGE_SHIFT;
> +				if (likely(index < size)) {
> +					buf[index++] = cpu_to_le64(addr);
> +				} else {
> +					spin_unlock_irqrestore(&zone->lock,
> +							       flags);
> +					return index;
> +				}
> +			}
> +		}
> +		spin_unlock_irqrestore(&zone->lock, flags);
> +	}
> +
> +	return index;
> +}

I wonder if (to address Michael's concern), you shouldn't instead use
the first free chunk of pages to return the addresses of all the pages.
ie something like this:

	__le64 *ret = NULL;
	unsigned int max = (PAGE_SIZE << order) / sizeof(__le64);

	for_each_populated_zone(zone) {
		spin_lock_irq(&zone->lock);
		for (mt = 0; mt < MIGRATE_TYPES; mt++) {
			list = &zone->free_area[order].free_list[mt];
			list_for_each_entry_safe(page, list, lru, ...) {
				if (index == size)
					break;
				addr = page_to_pfn(page) << PAGE_SHIFT;
				if (!ret) {
					list_del(...);
					ret = addr;
				}
				ret[index++] = cpu_to_le64(addr);
			}
		}
		spin_unlock_irq(&zone->lock);
	}

	return ret;
}

You'll need to return the page to the freelist afterwards, but free_pages()
should take care of that.