Re: [PATCH net-next v8 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Yunsheng Lin]

On 2023/9/15 16:28, Jesper Dangaard Brouer wrote:
> Hi Lin,
> 
> This looks reasonable, but given you are changing struct-page
> (include/linux/mm_types.h) we need to MM-list <linux-mm@xxxxxxxxx>.
> Also Cc Wilcox.
> 
> I think it was Ilias and Duyck that validated the assumptions, last time
> this patch was discussed. Thus I want to see their review before this is
> applied.

FWIW, PAGE_SIZE aligned buffer being PAGE_SIZE aligned in DMA is
validated by Duyck:
https://lore.kernel.org/all/CAKgT0UfeUAUQpEffxnkc+gzXsjOrHkuMgxU_Aw0VXSJYKzaovQ@xxxxxxxxxxxxxx/

And I had done researching to find out there seems to be no combination of
the above arch with an address space >16TB:
https://lore.kernel.org/all/2b570282-24f8-f23b-1ff7-ad836794baa9@xxxxxxxxxx/

> 
> -Jesper
> 
> On 12/09/2023 10.31, Yunsheng Lin wrote:
>> Currently page_pool_alloc_frag() is not supported in 32-bit
>> arch with 64-bit DMA because of the overlap issue between
>> pp_frag_count and dma_addr_upper in 'struct page' for those
>> arches, which seems to be quite common, see [1], which means
>> driver may need to handle it when using frag API.
>>
>> It is assumed that the combination of the above arch with an
>> address space >16TB does not exist, as all those arches have
>> 64b equivalent, it seems logical to use the 64b version for a
>> system with a large address space. It is also assumed that dma
>> address is page aligned when we are dma mapping a page aliged
>> buffer, see [2].
>>
>> That means we're storing 12 bits of 0 at the lower end for a
>> dma address, we can reuse those bits for the above arches to
>> support 32b+12b, which is 16TB of memory.
>>
>> If we make a wrong assumption, a warning is emitted so that
>> user can report to us.
>>
>> 1. https://lore.kernel.org/all/20211117075652.58299-1-linyunsheng@xxxxxxxxxx/
>> 2. https://lore.kernel.org/all/20230818145145.4b357c89@xxxxxxxxxx/
>>
>> Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> CC: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
>> CC: Alexander Duyck <alexander.duyck@xxxxxxxxx>
>> CC: Liang Chen <liangchen.linux@xxxxxxxxx>
>> CC: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>> CC: Guillaume Tucker <guillaume.tucker@xxxxxxxxxxxxx>
>> ---
>>   include/linux/mm_types.h        | 13 +------------
>>   include/net/page_pool/helpers.h | 20 ++++++++++++++------
>>   net/core/page_pool.c            | 14 +++++++++-----
>>   3 files changed, 24 insertions(+), 23 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 36c5b43999e6..74b49c4c7a52 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -125,18 +125,7 @@ struct page {
>>               struct page_pool *pp;
>>               unsigned long _pp_mapping_pad;
>>               unsigned long dma_addr;
>> -            union {
>> -                /**
>> -                 * dma_addr_upper: might require a 64-bit
>> -                 * value on 32-bit architectures.
>> -                 */
>> -                unsigned long dma_addr_upper;
>> -                /**
>> -                 * For frag page support, not supported in
>> -                 * 32-bit architectures with 64-bit DMA.
>> -                 */
>> -                atomic_long_t pp_frag_count;
>> -            };
>> +            atomic_long_t pp_frag_count;
>>           };
>>           struct {    /* Tail pages of compound page */
>>               unsigned long compound_head;    /* Bit zero is set */
>> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
>> index 94231533a369..8e1c85de4995 100644
>> --- a/include/net/page_pool/helpers.h
>> +++ b/include/net/page_pool/helpers.h
>> @@ -197,7 +197,7 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>>       page_pool_put_full_page(pool, page, true);
>>   }
>>   -#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT    \
>> +#define PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA    \
>>           (sizeof(dma_addr_t) > sizeof(unsigned long))
>>     /**
>> @@ -211,17 +211,25 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>>   {
>>       dma_addr_t ret = page->dma_addr;
>>   -    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> -        ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
>> +    if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA)
>> +        ret <<= PAGE_SHIFT;
>>         return ret;
>>   }
>>   -static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>> +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>>   {
>> +    if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA) {
>> +        page->dma_addr = addr >> PAGE_SHIFT;
>> +
>> +        /* We assume page alignment to shave off bottom bits,
>> +         * if this "compression" doesn't work we need to drop.
>> +         */
>> +        return addr != (dma_addr_t)page->dma_addr << PAGE_SHIFT;
>> +    }
>> +
>>       page->dma_addr = addr;
>> -    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
>> -        page->dma_addr_upper = upper_32_bits(addr);
>> +    return false;
>>   }
>>     static inline bool page_pool_put(struct page_pool *pool)
>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>> index 77cb75e63aca..8a9868ea5067 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -211,10 +211,6 @@ static int page_pool_init(struct page_pool *pool,
>>            */
>>       }
>>   -    if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
>> -        pool->p.flags & PP_FLAG_PAGE_FRAG)
>> -        return -EINVAL;
>> -
>>   #ifdef CONFIG_PAGE_POOL_STATS
>>       pool->recycle_stats = alloc_percpu(struct page_pool_recycle_stats);
>>       if (!pool->recycle_stats)
>> @@ -359,12 +355,20 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
>>       if (dma_mapping_error(pool->p.dev, dma))
>>           return false;
>>   -    page_pool_set_dma_addr(page, dma);
>> +    if (page_pool_set_dma_addr(page, dma))
>> +        goto unmap_failed;
>>         if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>>           page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
>>         return true;
>> +
>> +unmap_failed:
>> +    WARN_ON_ONCE("unexpected DMA address, please report to netdev@");
>> +    dma_unmap_page_attrs(pool->p.dev, dma,
>> +                 PAGE_SIZE << pool->p.order, pool->p.dma_dir,
>> +                 DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
>> +    return false;
>>   }
>>     static void page_pool_set_pp_info(struct page_pool *pool,
> 
> .
>