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Date:   Wed, 13 Jul 2022 12:07:39 +0200 (CEST)
From:   Christoph Lameter <cl@gentwo.de>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
cc:     Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Joe Perches <joe@perches.com>,
        Vasily Averin <vasily.averin@linux.dev>,
        Matthew WilCox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Marco Elver <elver@google.com>
Subject: Re: [PATCH 16/16] mm/sl[au]b: check if large object is valid in
 __ksize()
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On Wed, 13 Jul 2022, Hyeonggon Yoo wrote:

> > Why return 0 if there is an error and why bother the callers with these
> > checks. BUG()?
>
> I thought BUG should be used when there is no other solution.

Spurios returns of 0 that the caller has to check for is a solution?

> > I guess this is an error since the order-0 page cannot come from slab
> > allocations.
>
> comment in ksize() says:
> 	"The caller must guarantee that objp points to a valid object
> 	previously allocated with either kmalloc() or kmem_cache_alloc()."
>
> It should not be used on order-0 page that is not allocated from slab. No?

I guess we would need to check. Code could exist that does this.

Getting a 0 size would be surprising too here. BUG()? Or WARN() and return
PAGE_SIZE.