Messages in this thread | | | Date | Thu, 08 Dec 2011 11:24:25 -0500 | From | KOSAKI Motohiro <> | Subject | Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages? |
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(12/5/11 11:14 AM), Michal Hocko wrote: > 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@suse.cz> > 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@gmail.com> > Signed-off-by: Michal Hocko<mhocko@suse.cz> > --- > 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; > if (alloc_flags& ALLOC_HIGH) > min -= min / 2; > if (alloc_flags& ALLOC_HARDER)
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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