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

[Michal Hocko]

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 <address@hidden>

> 
> > Signed-off-by: Wei Wang <address@hidden>
> > Signed-off-by: Liang Li <address@hidden>
> > ---
> >  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 address@hidden  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"address@hidden";> address@hidden </a>
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs