Re: Bogus struct page layout on 32-bit

From: Grygorii Strashko
Date: Fri Apr 16 2021 - 05:27:55 EST

Hi Ilias, All,

On 10/04/2021 11:52, Ilias Apalodimas wrote:
+CC Grygorii for the cpsw part as Ivan's email is not valid anymore

Thanks for catching this. Interesting indeed...

On Sat, 10 Apr 2021 at 09:22, Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote:

On Sat, 10 Apr 2021 03:43:13 +0100
Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

On Sat, Apr 10, 2021 at 06:45:35AM +0800, kernel test robot wrote:
include/linux/mm_types.h:274:1: error: static_assert failed due to requirement '__builtin_offsetof(struct page, lru) == __builtin_offsetof(struct folio, lru)' "offsetof(struct page, lru) == offsetof(struct folio, lru)"
FOLIO_MATCH(lru, lru);
include/linux/mm_types.h:272:2: note: expanded from macro 'FOLIO_MATCH'
static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl))

Well, this is interesting. pahole reports:

struct page {
long unsigned int flags; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
union {
struct {
struct list_head lru; /* 8 8 */
struct folio {
union {
struct {
long unsigned int flags; /* 0 4 */
struct list_head lru; /* 4 8 */

so this assert has absolutely done its job.

But why has this assert triggered? Why is struct page layout not what
we thought it was? Turns out it's the dma_addr added in 2019 by commit
c25fff7171be ("mm: add dma_addr_t to struct page"). On this particular
config, it's 64-bit, and ppc32 requires alignment to 64-bit. So
the whole union gets moved out by 4 bytes.

Argh, good that you are catching this!

Unfortunately, we can't just fix this by putting an 'unsigned long pad'
in front of it. It still aligns the entire union to 8 bytes, and then
it skips another 4 bytes after the pad.

We can fix it like this ...

+++ b/include/linux/mm_types.h
@@ -96,11 +96,12 @@ struct page {
unsigned long private;
struct { /* page_pool used by netstack */
+ unsigned long _page_pool_pad;

I'm fine with this pad. Matteo is currently proposing[1] to add a 32-bit
value after @dma_addr, and he could use this area instead.


When adding/changing this, we need to make sure that it doesn't overlap
member @index, because network stack use/check page_is_pfmemalloc().
As far as my calculations this is safe to add. I always try to keep an
eye out for this, but I wonder if we could have a build check like yours.

* @dma_addr: might require a 64-bit value even on
* 32-bit architectures.
- dma_addr_t dma_addr;
+ dma_addr_t dma_addr __packed;
struct { /* slab, slob and slub */
union {

but I don't know if GCC is smart enough to realise that dma_addr is now
on an 8 byte boundary and it can use a normal instruction to access it,
or whether it'll do something daft like use byte loads to access it.

We could also do:

+ dma_addr_t dma_addr __packed __aligned(sizeof(void *));

and I see pahole, at least sees this correctly:

struct {
long unsigned int _page_pool_pad; /* 4 4 */
dma_addr_t dma_addr __attribute__((__aligned__(4))); /* 8 8 */
} __attribute__((__packed__)) __attribute__((__aligned__(4)));

This presumably affects any 32-bit architecture with a 64-bit phys_addr_t
/ dma_addr_t. Advice, please?

I'm not sure that the 32-bit behavior is with 64-bit (dma) addrs.

I don't have any 32-bit boards with 64-bit DMA. Cc. Ivan, wasn't your
board (572x ?) 32-bit with driver 'cpsw' this case (where Ivan added
XDP+page_pool) ?

Sry, for delayed reply.

The TI platforms am3/4/5 (cpsw) and Keystone 2 (netcp) can do only 32bit DMA even in case of LPAE (dma-ranges are used).
Originally, as I remember, CONFIG_ARCH_DMA_ADDR_T_64BIT has not been selected for the LPAE case
on TI platforms and the fact that it became set is the result of multi-paltform/allXXXconfig/DMA
optimizations and unification.
(just checked - not set in 4.14)

Probable commit 4965a68780c5 ("arch: define the ARCH_DMA_ADDR_T_64BIT config symbol in lib/Kconfig").

The TI drivers have been updated, finally to accept ARCH_DMA_ADDR_T_64BIT=y by using things like (__force u32)
for example.

Honestly, I've done sanity check of CPSW with LPAE=y (ARCH_DMA_ADDR_T_64BIT=y) very long time ago.

Best regards,