On 2021/7/11 0:55, Alexander Duyck wrote:
> On Sat, Jul 10, 2021 at 12:44 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>>
>> As suggested by Alexander, "A DMA mapping should be page
>> aligned anyway so the lower 12 bits would be reserved 0",
>> so it might make more sense to repurpose the lower 12 bits
>> of the dma address to store the pagecnt_bias for elevated
>> refcnt case in page pool.
>>
>> As newly added page_pool_get_pagecnt_bias() may be called
>> outside of the softirq context, so annotate the access to
>> page->dma_addr[0] with READ_ONCE() and WRITE_ONCE().
>>
>> Other three interfaces using page->dma_addr[0] is only called
>> in the softirq context during normal rx processing, hopefully
>> the barrier in the rx processing will ensure the correct order
>> between getting and setting pagecnt_bias.
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> ---
>> include/net/page_pool.h | 24 ++++++++++++++++++++++--
>> 1 file changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>> index 8d7744d..5746f17 100644
>> --- a/include/net/page_pool.h
>> +++ b/include/net/page_pool.h
>> @@ -200,7 +200,7 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>>
>> static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>> {
>> - dma_addr_t ret = page->dma_addr[0];
>> + dma_addr_t ret = READ_ONCE(page->dma_addr[0]) & PAGE_MASK;
>> if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
>> return ret;
>> @@ -208,11 +208,31 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>>
>> static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
>> {
>> - page->dma_addr[0] = addr;
>> + unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
>> +
>> + dma_addr_0 &= ~PAGE_MASK;
>> + dma_addr_0 |= (addr & PAGE_MASK);
>
> So rather than doing all this testing and clearing it would probably
> be better to add a return value to the function and do something like:
>
> if (WARN_ON(dma_addr_0 & ~PAGE_MASK))
> return -1;
>
> That way you could have page_pool_dma_map unmap, free the page, and
> return false indicating that the DMA mapping failed with a visible
> error in the event that our expectionat that the dma_addr is page
> aligned is ever violated.
I suppose the above is based on that page_pool_set_dma_addr() is called
only once before page_pool_set_pagecnt_bias(), right? so we could:
static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
{
if (WARN_ON(dma_addr_0 & ~PAGE_MASK))
return false;
page->dma_addr[0] = addr;
if (sizeof(dma_addr_t) > sizeof(unsigned long))
page->dma_addr[1] = upper_32_bits(addr);
return true;
}
>
>> + WRITE_ONCE(page->dma_addr[0], dma_addr_0);
>> +
>> if (sizeof(dma_addr_t) > sizeof(unsigned long))
>> page->dma_addr[1] = upper_32_bits(addr);
>> }
>>
>> +static inline int page_pool_get_pagecnt_bias(struct page *page)
>> +{
>> + return (READ_ONCE(page->dma_addr[0]) & ~PAGE_MASK);
>
> You don't need the parenthesis around the READ_ONCE and PAGE_MASK.
ok.
>
>> +}
>> +
>> +static inline void page_pool_set_pagecnt_bias(struct page *page, int bias)
>> +{
>> + unsigned long dma_addr_0 = READ_ONCE(page->dma_addr[0]);
>> +
>> + dma_addr_0 &= PAGE_MASK;
>> + dma_addr_0 |= (bias & ~PAGE_MASK);
>> +
>> + WRITE_ONCE(page->dma_addr[0], dma_addr_0);
>> +}
>> +
>> static inline bool is_page_pool_compiled_in(void)
>> {
>> #ifdef CONFIG_PAGE_POOL
>> --
>> 2.7.4
>>
> .
>
