Re: [PATCH net-next 3/7] pool_pool: avoid calling compound_head() for skb frag page

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/9/23 19:47, Ilias Apalodimas wrote:
> On Thu, 23 Sept 2021 at 14:24, Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> On 2021/9/23 16:33, Ilias Apalodimas wrote:
>>> On Wed, Sep 22, 2021 at 05:41:27PM +0800, Yunsheng Lin wrote:
>>>> As the pp page for a skb frag is always a head page, so make
>>>> sure skb_pp_recycle() passes a head page to avoid calling
>>>> compound_head() for skb frag page case.
>>>
>>> Doesn't that rely on the driver mostly (i.e what's passed in skb_frag_set_page() ?
>>> None of the current netstack code assumes bv_page is the head page of a
>>> compound page.  Since our page_pool allocator can will allocate compound
>>> pages for order > 0,  why should we rely on it ?
>>
>> As the page pool alloc function return 'struct page *' to the caller, which
>> is the head page of a compound pages for order > 0, so I assume the caller
>> will pass that to skb_frag_set_page().
> 
> Yea that's exactly the assumption I was afraid of.
> Sure not passing the head page might seem weird atm and the assumption
> stands, but the point is we shouldn't blow up the entire network stack
> if someone does that eventually.
> 
>>
>> For non-pp page, I assume it is ok whether the page is a head page or tail
>> page, as the pp_magic for both of them are not set with PP_SIGNATURE.
> 
> Yea that's true, although we removed the checking for coalescing
> recyclable and non-recyclable SKBs,   the next patch first checks the
> signature before trying to do anything with the skb.
> 
>>
>> Or should we play safe here, and do the trick as skb_free_head() does in
>> patch 6?
> 
> I don't think the &1 will even be measurable,  so I'd suggest just
> dropping this and play safe?

I am not sure what does '&1' mean above.

The one thing I am not sure about the trick done in patch 6 is that
if __page_frag_cache_drain() is right API to use here, I used it because
it is the only API that is expecting a head page.

> 
> Cheers
> /Ilias
>>
>>>
>>> Thanks
>>> /Ilias
>>>>
>>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>>>> ---
>>>>  include/linux/skbuff.h | 2 +-
>>>>  net/core/page_pool.c   | 2 --
>>>>  2 files changed, 1 insertion(+), 3 deletions(-)
>>>>
>>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>>> index 6bdb0db3e825..35eebc2310a5 100644
>>>> --- a/include/linux/skbuff.h
>>>> +++ b/include/linux/skbuff.h
>>>> @@ -4722,7 +4722,7 @@ static inline bool skb_pp_recycle(struct sk_buff *skb, void *data)
>>>>  {
>>>>      if (!IS_ENABLED(CONFIG_PAGE_POOL) || !skb->pp_recycle)
>>>>              return false;
>>>> -    return page_pool_return_skb_page(virt_to_page(data));
>>>> +    return page_pool_return_skb_page(virt_to_head_page(data));
>>>>  }
>>>>
>>>>  #endif      /* __KERNEL__ */
>>>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>>>> index f7e71dcb6a2e..357fb53343a0 100644
>>>> --- a/net/core/page_pool.c
>>>> +++ b/net/core/page_pool.c
>>>> @@ -742,8 +742,6 @@ bool page_pool_return_skb_page(struct page *page)
>>>>  {
>>>>      struct page_pool *pp;
>>>>
>>>> -    page = compound_head(page);
>>>> -
>>>>      /* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation
>>>>       * in order to preserve any existing bits, such as bit 0 for the
>>>>       * head page of compound page and bit 1 for pfmemalloc page, so
>>>> --
>>>> 2.33.0
>>>>
>>> .
>>>
> .
>