Re: [PATCH] mm: slub: reinitialize random sequence cache on slab object update

[vjitta]

On 2020-02-03 12:27, Vijayanand Jitta wrote:
> On 1/30/2020 11:58 PM, Christopher Lameter wrote:
> > On Thu, 30 Jan 2020, vjitta@xxxxxxxxxxxxxx wrote:
> >
> > > Random sequence cache is precomputed during slab object creation
> > > based up on the object size and no of objects per slab. These could
> > > be changed when flags like SLAB_STORE_USER, SLAB_POISON are updated
> > > from sysfs. So when shuffle_freelist is called during slab_alloc it
> >
> > Sorry no. That cannot happen. Changing the size of the slab is only
> > possible if no slab pages are allocated. Any sysfs changes that affect 
> > the
> > object size must fail if object and slab pages are already allocated.
> >
> > If you were able to change the object size then we need to prevent 
> > that
> > from happening.
>
> Yes, size of slab can't be changed after objects are allocated, that 
> holds
> true even with this change. Let me explain a bit more about the use 
> case here
>
> ZRAM compression uses the slub allocator, by enabling the slub debug 
> flags like
> SLAB_STORE_USER etc.. the memory consumption will rather be increased
> which doesn't
> serve the purpose of ZRAM compression. So, such flags are to be
> disabled before the
> allocations happen, this requires updation of random sequence cache as 
> object
> size and number of objects change after these flags are disabled.
>
> So, the sequence will be
>
> 1. Slab creation (this will set a precomputed random sequence cache)
> 2. Remove the debug flags
> 3. update the random sequence cache
> 4. Mount zram and then start using it for allocations.
>
> Thanks,
> Vijay

Waiting for your response.

Thanks,
Vijay