Re: [PATCH] slub: correctly bootstrap boot caches

Glauber Costa
2013-02-22T15:08:47Z

On 02/22/2013 07:00 PM, Christoph Lameter wrote:
> On Fri, 22 Feb 2013, Glauber Costa wrote:
>
>> Although not verified in practice, I also point out that it is not safe to scan
>> the full list only when debugging is on in this case. As unlikely as it is, it
>> is theoretically possible for the pages to be full. If they are, they will
>> become unreachable. Aside from scanning the full list, we also need to make
>> sure that the pages indeed sit in there: the easiest way to do it is to make
>> sure the boot caches have the SLAB_STORE_USER debug flag set.
>
> SLAB_STORE_USER typically increases the size of the managed object. It is
> not available when slab debugging is not compiled in. There is no list of
> full slab objects that is maintained in the non debug case and if the
> allocator is compiled without debug support also the code to manage full
> lists will not be present.
>
> Only one or two kmem_cache item is allocated in the bootstrap code and so
> far the size of the objects was signficantly smaller than page size. So
> the slab pages will be on the partial lists. Why are your slab management
> structures so large that a page can no longer contain multiple objects?
>
They are not.

As I've mentioned in the description, the real bug is from partial slabs
being temporarily in the cpu_slab during a recent allocation and
therefore unreachable through the partial list.

I've just read the code, and it seemed to me that theoretically that
could happen. I agree with you that this is an unlikely scenario and if
you prefer I can resend the patch without that part.

Would that be preferable ?