linux-kernel - Re: [PATCH v2 1/2] mm: krealloc: consider spare memory for __GFP

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Message-ID: <1f5ae113-3782-7351-ebec-c7f2e3cdb14a@google.com>
Date: Mon, 12 Aug 2024 19:32:08 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
cc: akpm@...ux-foundation.org, cl@...ux.com, penberg@...nel.org, 
    iamjoonsoo.kim@....com, vbabka@...e.cz, roman.gushchin@...ux.dev, 
    42.hyeyoo@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm: krealloc: consider spare memory for
 __GFP_ZERO

On Tue, 13 Aug 2024, Danilo Krummrich wrote:

> As long as krealloc() is called with __GFP_ZERO consistently, starting
> with the initial memory allocation, __GFP_ZERO should be fully honored.
> 
> However, if for an existing allocation krealloc() is called with a
> decreased size, it is not ensured that the spare portion the allocation
> is zeroed. Thus, if krealloc() is subsequently called with a larger size
> again, __GFP_ZERO can't be fully honored, since we don't know the
> previous size, but only the bucket size.
> 
> Example:
> 
> 	buf = kzalloc(64, GFP_KERNEL);
> 	memset(buf, 0xff, 64);
> 
> 	buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO);
> 
> 	/* After this call the last 16 bytes are still 0xff. */
> 	buf = krealloc(buf, 64, GFP_KERNEL | __GFP_ZERO);
> 
> Fix this, by explicitly setting spare memory to zero, when shrinking an
> allocation with __GFP_ZERO flag set or init_on_alloc enabled.
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Acked-by: David Rientjes <rientjes@...gle.com>