Re: [patch 1/4 v2] mm: exclude reserved pages from dirtyable memory

Minchan Kim
2011-09-28T04:56:06Z

Hi Hannes,

On Fri, Sep 23, 2011 at 04:38:17PM +0200, Johannes Weiner wrote:
> The amount of dirtyable pages should not include the full number of
> free pages: there is a number of reserved pages that the page
> allocator and kswapd always try to keep free.
>
> The closer (reclaimable pages - dirty pages) is to the number of
> reserved pages, the more likely it becomes for reclaim to run into
> dirty pages:
>
> +----------+ ---
> | anon | |
> +----------+ |
> | | |
> | | -- dirty limit new -- flusher new
> | file | | |
> | | | |
> | | -- dirty limit old -- flusher old
> | | |
> +----------+ --- reclaim
> | reserved |
> +----------+
> | kernel |
> +----------+
>
> This patch introduces a per-zone dirty reserve that takes both the
> lowmem reserve as well as the high watermark of the zone into account,
> and a global sum of those per-zone values that is subtracted from the
> global amount of dirtyable pages. The lowmem reserve is unavailable
> to page cache allocations and kswapd tries to keep the high watermark
> free. We don't want to end up in a situation where reclaim has to
> clean pages in order to balance zones.
>
> Not treating reserved pages as dirtyable on a global level is only a
> conceptual fix. In reality, dirty pages are not distributed equally
> across zones and reclaim runs into dirty pages on a regular basis.
>
> But it is important to get this right before tackling the problem on a
> per-zone level, where the distance between reclaim and the dirty pages
> is mostly much smaller in absolute numbers.
>
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> ---
> include/linux/mmzone.h | 6 ++++++
> include/linux/swap.h | 1 +
> mm/page-writeback.c | 6 ++++--
> mm/page_alloc.c | 19 +++++++++++++++++++
> 4 files changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 1ed4116..37a61e7 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -317,6 +317,12 @@ struct zone {
> */
> unsigned long lowmem_reserve[MAX_NR_ZONES];
>
> + /*
> + * This is a per-zone reserve of pages that should not be
> + * considered dirtyable memory.
> + */
> + unsigned long dirty_balance_reserve;
> +
> #ifdef CONFIG_NUMA
> int node;
> /*
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index b156e80..9021453 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -209,6 +209,7 @@ struct swap_list_t {
> /* linux/mm/page_alloc.c */
> extern unsigned long totalram_pages;
> extern unsigned long totalreserve_pages;
> +extern unsigned long dirty_balance_reserve;
> extern unsigned int nr_free_buffer_pages(void);
> extern unsigned int nr_free_pagecache_pages(void);
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index da6d263..c8acf8a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -170,7 +170,8 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
> &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
>
> x += zone_page_state(z, NR_FREE_PAGES) +
> - zone_reclaimable_pages(z);
> + zone_reclaimable_pages(z) -
> + zone->dirty_balance_reserve;
> }
> /*
> * Make sure that the number of highmem pages is never larger
> @@ -194,7 +195,8 @@ static unsigned long determine_dirtyable_memory(void)
> {
> unsigned long x;
>
> - x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> + x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
> + dirty_balance_reserve;
>
> if (!vm_highmem_is_dirtyable)
> x -= highmem_dirtyable_memory(x);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1dba05e..f8cba89 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -96,6 +96,14 @@ EXPORT_SYMBOL(node_states);
>
> unsigned long totalram_pages __read_mostly;
> unsigned long totalreserve_pages __read_mostly;
> +/*
> + * When calculating the number of globally allowed dirty pages, there
> + * is a certain number of per-zone reserves that should not be
> + * considered dirtyable memory. This is the sum of those reserves
> + * over all existing zones that contribute dirtyable memory.
> + */
> +unsigned long dirty_balance_reserve __read_mostly;
> +
> int percpu_pagelist_fraction;
> gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
>
> @@ -5076,8 +5084,19 @@ static void calculate_totalreserve_pages(void)
> if (max > zone->present_pages)
> max = zone->present_pages;
> reserve_pages += max;
> + /*
> + * Lowmem reserves are not available to
> + * GFP_HIGHUSER page cache allocations and
> + * kswapd tries to balance zones to their high
> + * watermark. As a result, neither should be
> + * regarded as dirtyable memory, to prevent a
> + * situation where reclaim has to clean pages
> + * in order to balance the zones.
> + */

Could you put Mel's description instead of it if you don't mind?
If I didn't see Mel's thing, maybe I wouldn't suggest but it looks
more easier to understand.

> + zone->dirty_balance_reserve = max;
> }
> }
> + dirty_balance_reserve = reserve_pages;
> totalreserve_pages = reserve_pages;
> }
>
> --
> 1.7.6.2
>

--
Kinds regards,
Minchan Kim