Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Dec 20, 2019 at 11:22:18AM +0100, Greg Kroah-Hartman wrote:
> On Fri, Dec 20, 2019 at 11:19:27AM +0100, Marc Gonzalez wrote:
> > 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" ?
> 
> Or you can not use the devm interface for lots of tiny allocations :)

Oh nevermind, "normal" kmalloc allocations are all aligned that way
anyway, so that's not going to solve anything, sorry.

greg k-h