RE: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()
Hi Marc,
We sort of expected something like that to happen at some point.
Funny enough it's been a year since my change was accepted in v4.20
and only now somebody noticed :)
Though quite a few questions below.
> 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.
Could you please elaborate a bit on mentioned paddings?
I may understand the first one for 128 bytes but where does the
second one for 256 bytes come from?
> Let's enforce the alignment only for devm_kmalloc().
Ok so for devm_kmalloc() we don't change anything, right?
We still add the same padding before real data array.
> ---
> 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.
Now probably I'm missing something but when do you expect to save something?
If those smaller allocations are done with devm_kmalloc() you aren't
saving anything.
> I suppose there's no easy solution, though.
Right! It took a while till I was able to propose something
people [almost silently] agreed with.
> ---
> 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)
Even given your update with:
--->8
#define DEVM_KMALLOC_PADDING_SIZE \
((ARCH_KMALLOC_MINALIGN - sizeof(struct devres)) % ARCH_KMALLOC_MINALIGN)
--->8
I don't think I understand why do you need that "% ARCH_KMALLOC_MINALIGN" part?
> 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;
This implementation gets ugly and potentially will lead to problems later
when people will start changing code here. Compared to that initially aligned by
the compiler dr->data looks much more foolproof.
> /* 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;
Ditto. But first I'd like to understand what are you trying to really do
with your change and then we'll see if there could be any better implementation.
-Alexey
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Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()
On 18/12/2019 15:20, Alexey Brodkin 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.
>
> Could you please elaborate a bit on mentioned paddings?
> I may understand the first one for 128 bytes but where does the
> second one for 256 bytes come from?
Sure thing.
struct devres {
struct devres_node node;
u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
};
struct devres_node = 3 pointers
kmalloc dishes out memory in multiples of ARCH_KMALLOC_MINALIGN bytes.
On arm64, ARCH_KMALLOC_MINALIGN = 128
(Everything written below assumes ARCH_KMALLOC_MINALIGN = 128)
In alloc_dr() we request sizeof(struct devres) + sizeof(data) from kmalloc.
sizeof(struct devres) = 128 because of the alignment directive.
I.e. the 'data' field is automatically padded to 128 by the compiler.
For most devm allocs (non-devm_kmalloc allocs), data is just 1 or 2 pointers.
So kmalloc(128 + 16) allocates 256 bytes.
>> Let's enforce the alignment only for devm_kmalloc().
>
> Ok so for devm_kmalloc() we don't change anything, right?
> We still add the same padding before real data array.
(My commit message probably requires improvement & refining.)
Yes, the objective of my patch is to keep the same behavior for
devm_kmalloc() while reverting to the old behavior for all other
uses of struct devres.
>> 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.
>
> Now probably I'm missing something but when do you expect to save something?
> If those smaller allocations are done with devm_kmalloc() you aren't
> saving anything.
With my patch, a "non-kmalloc" struct devres would take 128 bytes, instead
of 256.
>> I suppose there's no easy solution, though.
>
> Right! It took a while till I was able to propose something
> people [almost silently] agreed with.
I meant the wider subject of dynamic allocation granularity.
The 128-byte requirement is only for DMA. Some (most?) uses of kmalloc
are not for DMA. If the user could provide a flag ("this is to be used
for DMA") we could save lots of memory for small non-DMA allocs.
>> +#define DEVM_KMALLOC_PADDING_SIZE \
>> +(ARCH_KMALLOC_MINALIGN - sizeof(struct devres) % ARCH_KMALLOC_MINALIGN)
>
> Even given your update with:
> --->8
> #define DEVM_KMALLOC_PADDING_SIZE \
> ((ARCH_KMALLOC_MINALIGN - sizeof(struct devres)) % ARCH_KMALLOC_MINALIGN)
> --->8
> I don't think I understand why do you need that "% ARCH_KMALLOC_MINALIGN"
> part?
To handle the case where sizeof(struct devres) > ARCH_KMALLOC_MINALIGN
e.g ARCH_KMALLOC_MINALIGN = 8 and sizeof(struct devres) = 12
>> +/* Add enough padding to provide a DMA-safe address */
>> +size += DEVM_KMALLOC_PADDING_SIZE;
>
> This implementation gets ugly and potentially will lead to problems later
> when people will start changing code here. Compared to that initially aligned
> by
> the compiler dr->data looks much more foolproof.
Yes, it's better to let the compiler handle the padding... But, we don't
want any padding in the non-devm_kmalloc use-case.
We could add a pointer to the data field, but arches with small
ARCH_KMALLOC_MINALIGN
will have to pay the size increase, which doesn't seem fair to them (x86,
amd64).
>> @@ -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;
>
> Ditto. But first I'd like to understand what are you trying to really do
> with your change and then we'll see if there could be any better
> implementation.
Basically, every call to devres_alloc() or devm_add_action() allocates
256 bytes instead of 128. A typical arm64 system will call these
thousands of times during driver probe.
Regards.
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[PATCH v17 02/23] arc: mm: Add p?d_leaf() definitions
walk_page_range() is going to be allowed to walk page tables other than
those of user space. For this it needs to know when it has reached a
'leaf' entry in the page tables. This information will be provided by the
p?d_leaf() functions/macros.
For arc, we only have two levels, so only pmd_leaf() is needed.
CC: Vineet Gupta
CC: linux-snps-arc@lists.infradead.org
Acked-by: Vineet Gupta
Signed-off-by: Steven Price
---
arch/arc/include/asm/pgtable.h | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arc/include/asm/pgtable.h b/arch/arc/include/asm/pgtable.h
index 9019ed9f9c94..12be7e1b7cc0 100644
--- a/arch/arc/include/asm/pgtable.h
+++ b/arch/arc/include/asm/pgtable.h
@@ -273,6 +273,7 @@ static inline void pmd_set(pmd_t *pmdp, pte_t *ptep)
#define pmd_none(x)(!pmd_val(x))
#definepmd_bad(x) ((pmd_val(x) & ~PAGE_MASK))
#define pmd_present(x) (pmd_val(x))
+#define pmd_leaf(x)(pmd_val(x) & _PAGE_HW_SZ)
#define pmd_clear(xp) do { pmd_val(*(xp)) = 0; } while (0)
#define pte_page(pte) pfn_to_page(pte_pfn(pte))
--
2.20.1
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