Messages in this thread | | | Subject | Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc() | From | Marc Gonzalez <> | Date | Fri, 20 Dec 2019 11:19:27 +0100 |
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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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