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Date:   Fri, 17 Apr 2020 15:59:00 +0000 (UTC)
From:   Christopher Lameter <cl@linux.com>
To:     Bernard Zhao <bernard@vivo.com>
cc:     Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel@vivo.com
Subject: Re: [PATCH] kmalloc_index optimization(add kmalloc max size check)
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On Fri, 17 Apr 2020, Bernard Zhao wrote:

> kmalloc size should never exceed KMALLOC_MAX_SIZE.
> kmalloc_index realise if size is exceed KMALLOC_MAX_SIZE, e.g 64M,
> kmalloc_index just return index 26, but never check with OS`s max
> kmalloc config KMALLOC_MAX_SIZE. This index`s kmalloc caches maybe
> not create in function create_kmalloc_caches.
> We can throw an warninginfo in kmalloc at the beginning, instead of
> being guaranteed by the buddy alloc behind.

kmalloc_index(0 already bugs if the allocation is more than 64M


...

   if (size <=  64 * 1024 * 1024) return 26;
        BUG();


You could modify that to check for KMALLOC_MAX_SIZE with some more
conditionals but then kmalloc_index) is written so that the compiler gets
constant folding right.

If you have a patch like that then please verify that all c compilers in
use perform correct constant folding and do not add unnecessary code.