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

Minchan Kim
2011-09-28T18:35:31Z

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