Re: [PATCH v12 6/8] mm: support reporting free page blocks

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jul 14, 2017 at 02:54:30PM +0200, Michal Hocko wrote:
> On Fri 14-07-17 14:30:23, Michal Hocko wrote:
> > On Wed 12-07-17 20:40:19, Wei Wang wrote:
> > > This patch adds support for reporting blocks of pages on the free list
> > > specified by the caller.
> > > 
> > > As pages can leave the free list during this call or immediately
> > > afterwards, they are not guaranteed to be free after the function
> > > returns. The only guarantee this makes is that the page was on the free
> > > list at some point in time after the function has been invoked.
> > > 
> > > Therefore, it is not safe for caller to use any pages on the returned
> > > block or to discard data that is put there after the function returns.
> > > However, it is safe for caller to discard data that was in one of these
> > > pages before the function was invoked.
> > 
> > I do not understand what is the point of such a function and how it is
> > used because the patch doesn't give us any user (I haven't checked other
> > patches yet).
> > 
> > But just from the semantic point of view this sounds like a horrible
> > idea. The only way to get a free block of pages is to call the page
> > allocator. I am tempted to give it Nack right on those grounds but I
> > would like to hear more about what you actually want to achieve.
> 
> OK, so I gave it another thought and giving a page which is still on the
> free list to a random module is just a free ticket to a disaster.
> Nacked-by: Michal Hocko <mhocko@suse.com>

I agree it should be EXPORT_SYMBOL_GPL. Too much power
to give to non-GPL modules.

But pls take a look at the explanation I posted.  Any kind of hypervisor
hinting will need to do this by definition - best we can do is keep a
lock while we do this.

> > 
> > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > > ---
> > >  include/linux/mm.h |  5 +++
> > >  mm/page_alloc.c    | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 101 insertions(+)
> > > 
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 46b9ac5..76cb433 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -1835,6 +1835,11 @@ 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);
> > >  
> > > +#if IS_ENABLED(CONFIG_VIRTIO_BALLOON)
> > > +extern int report_unused_page_block(struct zone *zone, unsigned int order,
> > > +				    unsigned int migratetype,
> > > +				    struct page **page);
> > > +#endif
> > >  /*
> > >   * 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 64b7d82..8b3c9dd 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4753,6 +4753,102 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> > >  	show_swap_cache_info();
> > >  }
> > >  
> > > +#if IS_ENABLED(CONFIG_VIRTIO_BALLOON)
> > > +
> > > +/*
> > > + * Heuristically get a page block in the system that is unused.
> > > + * It is possible that pages from the page block are used immediately after
> > > + * report_unused_page_block() returns. It is the caller's responsibility
> > > + * to either detect or prevent the use of such pages.
> > > + *
> > > + * The free list to check: zone->free_area[order].free_list[migratetype].
> > > + *
> > > + * If the caller supplied page block (i.e. **page) is on the free list, offer
> > > + * the next page block on the list to the caller. Otherwise, offer the first
> > > + * page block on the list.
> > > + *
> > > + * Note: it is not safe for caller to use any pages on the returned
> > > + * block or to discard data that is put there after the function returns.
> > > + * However, it is safe for caller to discard data that was in one of these
> > > + * pages before the function was invoked.
> > > + *
> > > + * Return 0 when a page block is found on the caller specified free list.
> > > + */
> > > +int report_unused_page_block(struct zone *zone, unsigned int order,
> > > +			     unsigned int migratetype, struct page **page)
> > > +{
> > > +	struct zone *this_zone;
> > > +	struct list_head *this_list;
> > > +	int ret = 0;
> > > +	unsigned long flags;
> > > +
> > > +	/* Sanity check */
> > > +	if (zone == NULL || page == NULL || order >= MAX_ORDER ||
> > > +	    migratetype >= MIGRATE_TYPES)
> > > +		return -EINVAL;
> > > +
> > > +	/* Zone validity check */
> > > +	for_each_populated_zone(this_zone) {
> > > +		if (zone == this_zone)
> > > +			break;
> > > +	}
> > > +
> > > +	/* Got a non-existent zone from the caller? */
> > > +	if (zone != this_zone)
> > > +		return -EINVAL;
> > 
> > Huh, what do you check for here? Why don't you simply
> > populated_zone(zone)?
> > 
> > > +
> > > +	spin_lock_irqsave(&this_zone->lock, flags);
> > > +
> > > +	this_list = &zone->free_area[order].free_list[migratetype];
> > > +	if (list_empty(this_list)) {
> > > +		*page = NULL;
> > > +		ret = 1;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/* The caller is asking for the first free page block on the list */
> > > +	if ((*page) == NULL) {
> > > +		*page = list_first_entry(this_list, struct page, lru);
> > > +		ret = 0;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/*
> > > +	 * The page block passed from the caller is not on this free list
> > > +	 * anymore (e.g. a 1MB free page block has been split). In this case,
> > > +	 * offer the first page block on the free list that the caller is
> > > +	 * asking for.
> > > +	 */
> > > +	if (PageBuddy(*page) && order != page_order(*page)) {
> > > +		*page = list_first_entry(this_list, struct page, lru);
> > > +		ret = 0;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/*
> > > +	 * The page block passed from the caller has been the last page block
> > > +	 * on the list.
> > > +	 */
> > > +	if ((*page)->lru.next == this_list) {
> > > +		*page = NULL;
> > > +		ret = 1;
> > > +		goto out;
> > > +	}
> > > +
> > > +	/*
> > > +	 * Finally, fall into the regular case: the page block passed from the
> > > +	 * caller is still on the free list. Offer the next one.
> > > +	 */
> > > +	*page = list_next_entry((*page), lru);
> > > +	ret = 0;
> > > +out:
> > > +	spin_unlock_irqrestore(&this_zone->lock, flags);
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(report_unused_page_block);
> > > +
> > > +#endif
> > > +
> > >  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
> > >  {
> > >  	zoneref->zone = zone;
> > > -- 
> > > 2.7.4
> > > 
> > > --
> > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > the body to majordomo@kvack.org.  For more info on Linux MM,
> > > see: http://www.linux-mm.org/ .
> > > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> -- 
> Michal Hocko
> SUSE Labs

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