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SubjectRe: Possible partial miss in pages needed for zone's memory map?
On Mon, 19 May 2008 23:19:37 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Sat, 17 May 2008 14:19:15 +0200 Johannes Weiner <hannes@saeurebad.de> wrote:
>
> > Hi,
> >
> > I stumbled over the following in the zone initialization code. Please
> > let me know if I overlooked something here.
> >
>
> hm, no takers. Let's add linux-mm.
>
> >
> > From: Johannes Weiner <hannes@saeurebad.de>
> > Subject: [PATCH] Don't drop a partial page in a zone's memory map size
> >
> > In a zone's present pages number, account for all pages occupied by the
> > memory map, including a partial.
> >
> > Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
> > ---
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3378,7 +3378,8 @@ static void __paginginit free_area_init_
> > * is used by this zone for memmap. This affects the watermark
> > * and per-cpu initialisations
> > */
> > - memmap_pages = (size * sizeof(struct page)) >> PAGE_SHIFT;
> > + memmap_pages =
> > + PAGE_ALIGN(size * sizeof(struct page)) >> PAGE_SHIFT;
> > if (realsize >= memmap_pages) {
> > realsize -= memmap_pages;
> > printk(KERN_DEBUG
>
> I looked in there for 30 seconds and collapsed in confusion over which
> variables are in which units.
>
Hmm, size * sizeof(struct page) is multiple of PAGE_SIZE in many case.
Becasue "size" is always alinged to (1 << MAX_ORDER -1) (I believe...).

ex.) In x86 case,
(1 << (MAX_ORDER(11) - 1)) * 4 (sizeof(long)) = (1 << 12) = PAGE_SIZE.

But not sure on other archs with various params.
I think above fix is correct.

Thanks,
-Kame



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