RE: [PATCH v35 1/5] mm: support to get hints of free page blocks

["Wang, Wei W" <wei.w.wang@intel.com>]

On Tuesday, July 10, 2018 5:31 PM, Wang, Wei W wrote:
> Subject: [PATCH v35 1/5] mm: support to get hints of free page blocks
> 
> This patch adds support to get free page blocks from a free page list.
> The physical addresses of the blocks are stored to a list of buffers passed
> from the caller. The obtained free page blocks are hints about free pages,
> because there is no guarantee that they are still on the free page list after the
> function returns.
> 
> 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 hinted as
> free pages but are written after this function returns will be captured by the
> hypervisor, and they will be added to the next round of memory transfer.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> Signed-off-by: Liang Li <liang.z.li@intel.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
>  include/linux/mm.h |  3 ++
>  mm/page_alloc.c    | 98
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 101 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h index a0fbb9f..5ce654f
> 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2007,6 +2007,9 @@ extern void free_area_init(unsigned long *
> zones_size);  extern void free_area_init_node(int nid, unsigned long *
> zones_size,
>  		unsigned long zone_start_pfn, unsigned long *zholes_size);
> extern void free_initmem(void);
> +unsigned long max_free_page_blocks(int order); int
> +get_from_free_page_list(int order, struct list_head *pages,
> +			    unsigned int size, unsigned long *loaded_num);
> 
>  /*
>   * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 1521100..b67839b
> 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5043,6 +5043,104 @@ void show_free_areas(unsigned int filter,
> nodemask_t *nodemask)
>  	show_swap_cache_info();
>  }
> 
> +/**
> + * max_free_page_blocks - estimate the max number of free page blocks
> + * @order: the order of the free page blocks to estimate
> + *
> + * This function gives a rough estimation of the possible maximum
> +number of
> + * free page blocks a free list may have. The estimation works on an
> +assumption
> + * that all the system pages are on that list.
> + *
> + * Context: Any context.
> + *
> + * Return: The largest number of free page blocks that the free list can have.
> + */
> +unsigned long max_free_page_blocks(int order) {
> +	return totalram_pages / (1 << order);
> +}
> +EXPORT_SYMBOL_GPL(max_free_page_blocks);
> +
> +/**
> + * get_from_free_page_list - get hints of free pages from a free page
> +list
> + * @order: the order of the free page list to check
> + * @pages: the list of page blocks used as buffers to load the
> +addresses
> + * @size: the size of each buffer in bytes
> + * @loaded_num: the number of addresses loaded to the buffers
> + *
> + * This function offers hints about free pages. The addresses of free
> +page
> + * blocks are stored to the list of buffers passed from the caller.
> +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 a buffer 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.
> + *
> + * Context: Process context.
> + *
> + * Return: 0 if all the free page block addresses are stored to the buffers;
> + *         -ENOSPC if the buffers are not sufficient to store all the
> + *         addresses; or -EINVAL if an unexpected argument is received (e.g.
> + *         incorrect @order, empty buffer list).
> + */
> +int get_from_free_page_list(int order, struct list_head *pages,
> +			    unsigned int size, unsigned long *loaded_num) {


Hi Linus,

We  took your original suggestion - pass in pre-allocated buffers to load the addresses (now we use a list of pre-allocated page blocks as buffers). Hope that suggestion is still acceptable (the advantage of this method was explained here: https://lkml.org/lkml/2018/6/28/184).
Look forward to getting your feedback. Thanks.

Best,
Wei