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SubjectRe: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()
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On 17/12/2019 16:30, Marc Gonzalez wrote:

> Commit a66d972465d15 ("devres: Align data[] to ARCH_KMALLOC_MINALIGN")
> increased the alignment of devres.data unconditionally.
>
> Some platforms have very strict alignment requirements for DMA-safe
> addresses, e.g. 128 bytes on arm64. There, struct devres amounts to:
> 3 pointers + pad_to_128 + data + pad_to_256
> i.e. ~220 bytes of padding.
>
> Let's enforce the alignment only for devm_kmalloc().
>
> Suggested-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
> ---
> I had not been aware that dynamic allocation granularity on arm64 was
> 128 bytes. This means there's a lot of waste on small allocations.
> I suppose there's no easy solution, though.
> ---
> drivers/base/devres.c | 23 +++++++++++++----------
> 1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index 0bbb328bd17f..bf39188613d9 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -26,14 +26,7 @@ struct devres_node {
>
> struct devres {
> struct devres_node node;
> - /*
> - * Some archs want to perform DMA into kmalloc caches
> - * and need a guaranteed alignment larger than
> - * the alignment of a 64-bit integer.
> - * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same
> - * buffer alignment as if it was allocated by plain kmalloc().
> - */
> - u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
> + u8 data[];
> };
>
> struct devres_group {
> @@ -789,9 +782,16 @@ static void devm_kmalloc_release(struct device *dev, void *res)
> /* noop */
> }
>
> +#define DEVM_KMALLOC_PADDING_SIZE \
> + (ARCH_KMALLOC_MINALIGN - sizeof(struct devres) % ARCH_KMALLOC_MINALIGN)
> +
> static int devm_kmalloc_match(struct device *dev, void *res, void *data)
> {
> - return res == data;
> + /*
> + * 'res' is dr->data (not DMA-safe)
> + * 'data' is the hand-aligned address from devm_kmalloc
> + */
> + return res + DEVM_KMALLOC_PADDING_SIZE == data;
> }
>
> /**
> @@ -811,6 +811,9 @@ void * devm_kmalloc(struct device *dev, size_t size, gfp_t gfp)
> {
> struct devres *dr;
>
> + /* Add enough padding to provide a DMA-safe address */
> + size += DEVM_KMALLOC_PADDING_SIZE;
> +
> /* use raw alloc_dr for kmalloc caller tracing */
> dr = alloc_dr(devm_kmalloc_release, size, gfp, dev_to_node(dev));
> if (unlikely(!dr))
> @@ -822,7 +825,7 @@ void * devm_kmalloc(struct device *dev, size_t size, gfp_t gfp)
> */
> set_node_dbginfo(&dr->node, "devm_kzalloc_release", size);
> devres_add(dev, dr->data);
> - return dr->data;
> + return dr->data + DEVM_KMALLOC_PADDING_SIZE;
> }
> EXPORT_SYMBOL_GPL(devm_kmalloc);

Would anyone else have any suggestions, comments, insights, recommendations,
improvements, guidance, or wisdom? :-)

I keep thinking about the memory waste caused by the strict alignment requirement
on arm64. Is there a way to inspect how much memory has been requested vs how much
has been allocated? (Turning on SLAB DEBUG perhaps?)

Couldn't there be a kmalloc flag saying "this alloc will not require strict
alignment, so just give me something 8-byte aligned" ?

Regards.

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