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Subject: Re: [PATCH] mm: slub: reinitialize random sequence cache on slab
 object update
To:     Christopher Lameter <cl@linux.com>
Cc:     penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, vinmenon@codeaurora.org,
        kernel-team@android.com
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 <alpine.DEB.2.21.2001301826130.9861@www.lameter.com>
From:   Vijayanand Jitta <vjitta@codeaurora.org>
Message-ID: <7bf56496-7b8a-c60f-b261-9505068f9130@codeaurora.org>
Date:   Mon, 3 Feb 2020 12:27:18 +0530
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On 1/30/2020 11:58 PM, Christopher Lameter wrote:
> On Thu, 30 Jan 2020, vjitta@codeaurora.org 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