Re: [patch 1/9] GFP_ZERO fix

[Manfred Spraul]

Andrew Morton wrote:

> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>  
>
> >  #define GFP_LEVEL_MASK (__GFP_WAIT|__GFP_HIGH|__GFP_IO|__GFP_FS| \
> > -			__GFP_COLD|__GFP_NOWARN|__GFP_REPEAT| \
> > -			__GFP_NOFAIL|__GFP_NORETRY|__GFP_NO_GROW|__GFP_COMP)
> > +			__GFP_COLD|__GFP_NOWARN|__GFP_REPEAT|__GFP_NOFAIL| \
> > +			__GFP_NORETRY|__GFP_NO_GROW|__GFP_COMP|__GFP_ZERO)
> >    
>
> Passing GFP_ZERO into kmem_cache_alloc() is such a bizarre thing to do,
> perhaps a BUG is the correct response.
>
> I guess it could be argued that the kmem_cache_alloc() callers "knows" that
> the ctor will be zeroing out all the objects, but it would seem cleaner to
> me to pass the "you should use GFP_ZERO" hint into kmem_cache_create()
> rather than kmem_cache_alloc().
>  
Right now, slab is not really suitable for GFP_ZERO:
- if debug is enabled, then objects are definitively not 0-initialized.
- if a ctor is used for zero initialization, then objects would have to 
be zeroed before kmem_cache_free: The ctor is only called at object 
creation, not before object reuse. But memset(,0,) just before free 
would be a bit silly.

Probably a BUG_ON or WARN_ON should be added into kmem_flagcheck() and 
into kmem_cache_create().

Should I write a patch?
--
   Manfred

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