Re: [PATCH 5/8] mm: Introduce _slub_counter_t

[Matthew Wilcox <willy@infradead.org>]

On Tue, Dec 19, 2017 at 09:07:31AM +0100, Michal Hocko wrote:
> On Sat 16-12-17 08:44:22, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@microsoft.com>
> > 
> > Instead of putting the ifdef in the middle of the definition of struct
> > page, pull it forward to the rest of the ifdeffery around the SLUB
> > cmpxchg_double optimisation.
> > 
> > Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> 
> The definition of struct page looks better now. I think that slub.c
> needs some love as well. I haven't checked too deeply but it seems that
> it assumes counters to be unsigned long in some places. Maybe I've
> missed some ifdef-ery but using the native type would be much better

I may have missed something, but I checked its use of 'counters' while
I was working on this patch, and I didn't *see* a problem.  The only
problem I really see is that it uses a bitfield for { inuse, objects,
frozen } and if the architecture has big-endian bitfields, it's possible
that slub's counters might end up conflicting with the special values we
use for PageBuddy, PageKmemcg and PageBalloon.  I always get confused
by big endian, so I can't even figure out how likely it is ... would
'frozen' end up as bit 31?  And if so, would _mapcount have its sign
bit at bit 31, or at bit 7?

> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

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