Messages in this thread | | | Date | Thu, 8 Nov 2012 15:37:56 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] mm: Fix calculation of dirtyable memory |
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On Thu, 8 Nov 2012 15:25:35 -0800 Sonny Rao <sonnyrao@chromium.org> wrote:
> The system uses global_dirtyable_memory() to calculate > number of dirtyable pages/pages that can be allocated > to the page cache. A bug causes an underflow thus making > the page count look like a big unsigned number. This in turn > confuses the dirty writeback throttling to aggressively write > back pages as they become dirty (usually 1 page at a time). > > Fix is to ensure there is no underflow while doing the math. > > Signed-off-by: Sonny Rao <sonnyrao@chromium.org> > Signed-off-by: Puneet Kumar <puneetster@chromium.org> > --- > mm/page-writeback.c | 17 +++++++++++++---- > 1 files changed, 13 insertions(+), 4 deletions(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index 830893b..2a6356c 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -194,11 +194,19 @@ static unsigned long highmem_dirtyable_memory(unsigned long total) > unsigned long x = 0; > > for_each_node_state(node, N_HIGH_MEMORY) { > + unsigned long nr_pages; > struct zone *z = > &NODE_DATA(node)->node_zones[ZONE_HIGHMEM]; > > - x += zone_page_state(z, NR_FREE_PAGES) + > - zone_reclaimable_pages(z) - z->dirty_balance_reserve; > + nr_pages = zone_page_state(z, NR_FREE_PAGES) + > + zone_reclaimable_pages(z); > + /* > + * Unreclaimable memory (kernel memory or anonymous memory > + * without swap) can bring down the dirtyable pages below > + * the zone's dirty balance reserve. > + */ > + if (nr_pages >= z->dirty_balance_reserve) > + x += nr_pages - z->dirty_balance_reserve;
If the system has two nodes and one is below its dirty_balance_reserve, we could end up with something like:
x = 0; ... x += 1000; ... x += -100;
In this case, your fix would cause highmem_dirtyable_memory() to return 1000. Would it be better to instead return 900?
IOW, we instead add logic along the lines of
if ((long)x < 0) x = 0; return x;
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