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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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