Messages in this thread | | | Date | Thu, 9 Jun 2011 09:45:26 -0700 | From | Andrew Morton <> | Subject | Re: [slubllv7 05/17] mm: Rearrange struct page |
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On Thu, 09 Jun 2011 12:57:50 +0300 Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On 6/1/11 8:25 PM, Christoph Lameter wrote: > > We need to be able to use cmpxchg_double on the freelist and object count > > field in struct page. Rearrange the fields in struct page according to > > doubleword entities so that the freelist pointer comes before the counters. > > Do the rearranging with a future in mind where we use more doubleword > > atomics to avoid locking of updates to flags/mapping or lru pointers. > > > > Create another union to allow access to counters in struct page as a > > single unsigned long value. > > > > The doublewords must be properly aligned for cmpxchg_double to work. > > Sadly this increases the size of page struct by one word on some architectures. > > But as a resultpage structs are now cacheline aligned on x86_64.
Is it worth this cost?
> > Signed-off-by: Christoph Lameter<cl@linux.com> > > I'd like to queue this SLUB patch series to linux-next through slab.git. > Any NAKs or ACKs for this patch? > > > include/linux/mm_types.h | 89 +++++++++++++++++++++++++++++++---------------- > > 1 file changed, 60 insertions(+), 29 deletions(-) > > > > Index: linux-2.6/include/linux/mm_types.h > > =================================================================== > > --- linux-2.6.orig/include/linux/mm_types.h 2011-05-31 09:46:41.912987862 -0500 > > +++ linux-2.6/include/linux/mm_types.h 2011-05-31 09:46:44.282987846 -0500 > > @@ -30,52 +30,74 @@ struct address_space; > > * moment. Note that we have no way to track which tasks are using > > * a page, though if it is a pagecache page, rmap structures can tell us > > * who is mapping it. > > + * > > + * The objects in struct page are organized in double word blocks in > > + * order to allows us to use atomic double word operations on portions > > + * of struct page. That is currently only used by slub but the arrangement > > + * allows the use of atomic double word operations on the flags/mapping > > + * and lru list pointers also.
I don't really like the word "word" much. There's always uncertainty about whether thee and me are talking about the same thing. And perhaps one day words will be 64-bit.
So if we mean 32-bit, let's say 32-bit?
> > +/* > > + * If another subsystem starts using the double word pairing for atomic > > + * operations on struct page then it must change the #if to ensure > > + * proper alignment of the page struct. > > + */ > > +#if defined(CONFIG_SLUB)&& defined(CONFIG_CMPXCHG_LOCAL) > > + __attribute__((__aligned__(2*sizeof(unsigned long)))) > > +#endif
I guess we need a "hey, use __aligned" checkpatch rule.
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