Re: [PATCH v14 4/5] mm: support reporting free page blocks

Michael S. Tsirkin
2017-08-18T17:23:18Z

On Thu, Aug 17, 2017 at 11:26:55AM +0800, 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.
>
> 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: Michael S. Tsirkin <mst@redhat.com>
> ---
> include/linux/mm.h | 6 ++++++
> mm/page_alloc.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 50 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 46b9ac5..cd29b9f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1835,6 +1835,12 @@ 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);
>
> +extern void walk_free_mem_block(void *opaque1,
> + unsigned int min_order,
> + void (*visit)(void *opaque2,
> + unsigned long pfn,
> + unsigned long nr_pages));
> +
> /*
> * 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..a721a35 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4762,6 +4762,50 @@ 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
> + * @opaque1: the context passed from the caller
> + * @min_order: the minimum order of free lists to check
> + * @visit: the callback function given by the caller
> + *
> + * The function is used to walk through the free page blocks in the system,
> + * and each free page block is reported to the caller via the @visit callback.
> + * Please note:
> + * 1) The function is used to report hints of free pages, so the caller should
> + * not use those reported pages after the callback returns.
> + * 2) The callback is invoked with the zone->lock being held, so it should not
> + * block and should finish as soon as possible.
> + */
> +void walk_free_mem_block(void *opaque1,
> + unsigned int min_order,
> + void (*visit)(void *opaque2,

You can just avoid opaque2 completely I think, then opaque1 can
be renamed opaque.

> + unsigned long pfn,
> + unsigned long nr_pages))
> +{
> + struct zone *zone;
> + struct page *page;
> + struct list_head *list;
> + unsigned int order;
> + enum migratetype mt;
> + unsigned long pfn, flags;
> +
> + for_each_populated_zone(zone) {
> + for (order = MAX_ORDER - 1;
> + order < MAX_ORDER && order >= min_order; order--) {
> + for (mt = 0; 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);
> + visit(opaque1, pfn, 1 << order);

My only concern here is inability of callback to
1. break out of list
2. remove page from the list

So I would make the callback bool, and I would use
list_for_each_entry_safe.

> + }
> + spin_unlock_irqrestore(&zone->lock, flags);
> + }
> + }
> + }
> +}
> +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> +
> static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
> {
> zoneref->zone = zone;
> --
> 2.7.4