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mm: krealloc: clarify valid usage of __GFP_ZERO

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Properly document that if __GFP_ZERO logic is requested, callers must
ensure that, starting with the initial memory allocation, every subsequent
call to this API for the same memory allocation is flagged with
__GFP_ZERO.  Otherwise, it is possible that __GFP_ZERO is not fully
honored by this API.

Link: https://lkml.kernel.org/r/20240812223707.32049-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Danilo Krummrich 2024-08-13 00:34:35 +02:00 • committed by Andrew Morton
parent 1a83a716ec
commit 489a744e5f
2 changed files with 28 additions and 2 deletions
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10
include/linux/slab.h
View file

@ -733,6 +733,16 @@ static inline __alloc_size(1, 2) void *kmalloc_array_noprof(size_t n, size_t siz
* @new_n: new number of elements to alloc
* @new_size: new size of a single member of the array
* @flags: the type of memory to allocate (see kmalloc)
*
* If __GFP_ZERO logic is requested, callers must ensure that, starting with the
* initial memory allocation, every subsequent call to this API for the same
* memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
* __GFP_ZERO is not fully honored by this API.
*
* See krealloc_noprof() for further details.
*
* In any case, the contents of the object pointed to are preserved up to the
* lesser of the new and old sizes.
*/
static inline __realloc_size(2, 3) void * __must_check krealloc_array_noprof(void *p,
size_t new_n,

20
mm/slab_common.c
View file

@ -1301,11 +1301,27 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
* @new_size: how many bytes of memory are required.
* @flags: the type of memory to allocate.
*
* The contents of the object pointed to are preserved up to the
* lesser of the new and old sizes (__GFP_ZERO flag is effectively ignored).
* If @p is %NULL, krealloc() behaves exactly like kmalloc(). If @new_size
* is 0 and @p is not a %NULL pointer, the object pointed to is freed.
*
* If __GFP_ZERO logic is requested, callers must ensure that, starting with the
* initial memory allocation, every subsequent call to this API for the same
* memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
* __GFP_ZERO is not fully honored by this API.
*
* This is the case, since krealloc() only knows about the bucket size of an
* allocation (but not the exact size it was allocated with) and hence
* implements the following semantics for shrinking and growing buffers with
* __GFP_ZERO.
*
* new bucket
* 0 size size
* |--------|----------------|
* | keep | zero |
*
* In any case, the contents of the object pointed to are preserved up to the
* lesser of the new and old sizes.
*
* Return: pointer to the allocated memory or %NULL in case of error
*/
void *krealloc_noprof(const void *p, size_t new_size, gfp_t flags)