On Sat, 17 Aug 2013 15:31:23 -0400 Johannes Weiner <han...@cmpxchg.org> wrote:
> Previously, page cache radix tree nodes were freed after reclaim
> emptied out their page pointers. But now reclaim stores shadow
> entries in their place, which are only reclaimed when the inodes
> themselves are reclaimed. This is problematic for bigger files that
> are still in use after they have a significant amount of their cache
> reclaimed, without any of those pages actually refaulting. The shadow
> entries will just sit there and waste memory. In the worst case, the
> shadow entries will accumulate until the machine runs out of memory.
erk. This whole patch is overhead :(
> To get this under control, a list of inodes that contain shadow
> entries is maintained. If the global number of shadows exceeds a
> certain threshold, a shrinker is activated that reclaims old entries
> from the mappings. This is heavy-handed but it should not be a hot
> path and is mainly there to protect from accidentally/maliciously
> induced OOM kills. The global list is also not a problem because the
> modifications are very rare: inodes are added once in their lifetime
> when the first shadow entry is stored (i.e. the first page reclaimed)
> and lazily removed when the inode exits. Or if the shrinker removes
> all shadow entries.
>
> ...
>
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -417,6 +417,7 @@ struct address_space {
> /* Protected by tree_lock together with the radix tree */
> unsigned long nrpages; /* number of total pages */
> unsigned long nrshadows; /* number of shadow entries */
> + struct list_head shadow_list; /* list of mappings with
> shadows */
> pgoff_t writeback_index;/* writeback starts here */
> const struct address_space_operations *a_ops; /* methods */
> unsigned long flags; /* error bits/gfp mask */
There's another 16 bytes into the inode. Bad.
>
> ...
>
> +void workingset_shadows_inc(struct address_space *mapping)
> +{
> + might_lock(&shadow_lock);
> +
> + if (mapping->nrshadows == 0 && list_empty(&mapping->shadow_list)) {
> + spin_lock(&shadow_lock);
I can't work out whether or not shadow_lock is supposed to be irq-save.
Some places it is, others are unobvious.
> + list_add(&mapping->shadow_list, &shadow_mappings);
> + spin_unlock(&shadow_lock);
> + }
> +
> + mapping->nrshadows++;
> + this_cpu_inc(nr_shadows);
> +}
> +
>
> ...
>
> +static unsigned long get_nr_old_shadows(void)
> +{
> + unsigned long nr_max;
> + unsigned long nr;
> + long sum = 0;
> + int cpu;
> +
> + for_each_possible_cpu(cpu)
> + sum += per_cpu(nr_shadows, cpu);
Ouch, slow. shrink_slab() will call this repeatedly and scan_shadows()
calls it from a loop. Can we use something non-deathly-slow here?
Like percpu_counter_read_positive()?
> + nr = max(sum, 0L);
> +
> + /*
> + * Every shadow entry with a refault distance bigger than the
> + * active list is ignored and so NR_ACTIVE_FILE would be a
> + * reasonable ceiling. But scanning and shrinking shadow
> + * entries is quite expensive, so be generous.
> + */
> + nr_max = global_dirtyable_memory() * 4;
> +
> + if (nr <= nr_max)
> + return 0;
> + return nr - nr_max;
> +}
> +
> +static unsigned long scan_mapping(struct address_space *mapping,
> + unsigned long nr_to_scan)
Some methodological description would be useful.
> +{
> + unsigned long nr_scanned = 0;
> + struct radix_tree_iter iter;
> + void **slot;
> +
> + rcu_read_lock();
> +restart:
> + radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, 0) {
> + unsigned long nrshadows;
> + unsigned long distance;
> + struct zone *zone;
> + struct page *page;
> +
> + page = radix_tree_deref_slot(slot);
> + if (unlikely(!page))
> + continue;
> + if (!radix_tree_exception(page))
> + continue;
> + if (radix_tree_deref_retry(page))
> + goto restart;
> +
> + unpack_shadow(page, &zone, &distance);
> +
> + if (distance <= zone_page_state(zone, NR_ACTIVE_FILE))
> + continue;
> +
> + spin_lock_irq(&mapping->tree_lock);
> + if (radix_tree_delete_item(&mapping->page_tree,
> + iter.index, page)) {
> + inc_zone_state(zone, WORKINGSET_SHADOWS_RECLAIMED);
> + workingset_shadows_dec(mapping);
> + nr_scanned++;
> + }
> + nrshadows = mapping->nrshadows;
> + spin_unlock_irq(&mapping->tree_lock);
> +
> + if (nrshadows == 0)
> + break;
> +
> + if (--nr_to_scan == 0)
> + break;
> + }
> + rcu_read_unlock();
> +
> + return nr_scanned;
> +}
> +
>
> ...
>
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