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

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, Sep 28, 2011 at 09:50:54AM +0200, Johannes Weiner wrote:
> On Wed, Sep 28, 2011 at 01:55:51PM +0900, Minchan Kim wrote:
> > 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.
> 
> I changed it because it was already referring to allocation placement,
> but at the point in time where this comment is introduced there is no
> allocation placement based on dirty pages yet.

Right. at this point, you don't introduce allocation placement yet but
I knew about that and it seems I was too hasty.
But I hope you add a comment about allocation placement when you introduce it.
Of course, you added it in page_alloc.c but I like adding short summary comment
on field as Mel does. Adding short summary comment on field helps understanding
why the field is introduced without jumping in and out.

> 
> The other thing is that it said lowmem_reserves were respected to
> prevent increasing lowmem pressure, but lowmem is protected by the
> watermark checks during the allocation.  I took it into account to not
> end up with a number of dirtyable pages that is bigger than the amount
> of technically available page cache pages.  Otherwise, you could end
> up with all page cache pages in a zone dirtied at the time reclaim
> kicks in and we are back to square one.
> 
> Maybe you can fingerpoint to the part that is harder to understand so
> I can fix it?

I don't mean yours is hard to understand. It seems to be a preference.
Mel's explanation is more straightforward, I think.
He explained objective,  method and why we select the method in introduction briefly.
I like such summary on the field.

But as I said, it might be a preference so if you mind it, I don't insist on it.

-- 
Kind regards,
Minchan Kim