On Thu 08-12-11 10:38:39, Wang Sheng-Hui wrote: > Sorry, Michal. > > 2011/12/6 Michal Hocko <mhocko@xxxxxxx> > > > On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote: > > > In line 1459, we have "free_pages -= (1 << order) + 1;". > > > Suppose allocating one 0-order page, here we'll get > > > free_pages -= 1 + 1 > > > I wonder why there is a "+ 1"? > > > > Good spot. Check the patch bellow. > > --- > > From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001 > > From: Michal Hocko <mhocko@xxxxxxx> > > Date: Mon, 5 Dec 2011 17:04:23 +0100 > > Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok > > > > 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when > > memory is low] changed the form how free_pages is calculated but it > > forgot that we used to do free_pages - ((1 << order) - 1) so we ended up > > with off-by-two when calculating free_pages. > > > > Spotted-by: Wang Sheng-Hui <shhuiw@xxxxxxxxx> > > Signed-off-by: Michal Hocko <mhocko@xxxxxxx> > > --- > > mm/page_alloc.c | 2 +- > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 9dd443d..8a2f1b6 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int > > order, unsigned long mark, > > long min = mark; > > int o; > > > > - free_pages -= (1 << order) + 1; > > + free_pages -= (1 << order) - 1; > > > > I don't understand why there is additional "-1". > Use 0-order allocation as example: > 0-order page ---- one 4K page > free_pages should subtract 1. Here, free_pages will subtract 0? Check out all the conditions for free_pages... -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>