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Subject: Re: [PATCH rfc v2 3/5] page_pool: add page recycling support based on
 elevated refcnt
To:     Alexander Duyck <alexander.duyck@gmail.com>
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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <d45619d1-5235-9dd4-77aa-497fa1115c80@huawei.com>
Date:   Mon, 12 Jul 2021 10:06:01 +0800
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On 2021/7/11 1:31, Alexander Duyck wrote:
> On Sat, Jul 10, 2021 at 12:44 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
> <snip>
>> @@ -419,6 +471,20 @@ static __always_inline struct page *
>>  __page_pool_put_page(struct page_pool *pool, struct page *page,
>>                      unsigned int dma_sync_size, bool allow_direct)
>>  {
>> +       int bias = page_pool_get_pagecnt_bias(page);
>> +
>> +       /* Handle the elevated refcnt case first */
>> +       if (bias) {
>> +               /* It is not the last user yet */
>> +               if (!page_pool_bias_page_recyclable(page, bias))
>> +                       return NULL;
>> +
>> +               if (likely(!page_is_pfmemalloc(page)))
>> +                       goto recyclable;
>> +               else
>> +                       goto unrecyclable;
>> +       }
>> +
> 
> So this part is still broken. Anything that takes a reference to the
> page and holds it while this is called will cause it to break. For
> example with the recent fixes we put in place all it would take is a
> skb_clone followed by pskb_expand_head and this starts leaking memory.

Ok, it seems the fix is confilcting with the expectation this patch is
based, which is "the last user will always call page_pool_put_full_page()
in order to do the recycling or do the resource cleanup(dma unmaping..etc)
and freeing.".

As the user of the new skb after skb_clone() and pskb_expand_head() is
not aware of that their frag page may still be in the page pool after
the fix?

> 
> One of the key bits in order for pagecnt_bias to work is that you have
> to deduct the bias once there are no more parties using it. Otherwise
> you leave the reference count artificially inflated and the page will
> never be freed. It works fine for the single producer single consumer
> case but once you introduce multiple consumers this is going to fall
> apart.

It seems we have diffferent understanding about consumer, I treat the
above user of new skb after skb_clone() and pskb_expand_head() as the
consumer of the page pool too, so that new skb should keep the recycle
bit in order for that to happen.

If the semantic is "the new user of a page should not be handled by page
pool if page pool is not aware of the new user(the new user is added by
calling page allocator API instead of calling the page pool API, like the
skb_clone() and pskb_expand_head() above) ", I suppose I am ok with that
semantic too as long as the above semantic is aligned with the people
involved.

Also, it seems _refcount and dma_addr in "struct page" is in the same cache
line, which means there is already cache line bouncing already between _refcount
and dma_addr updating, so it may makes senses to only use bias to indicate
number of the page pool user for a page, instead of using "bias - page_ref_count",
as the page_ref_count is not reliable if somebody is using the page allocator API
directly.

And the trick part seems to be how to make the bias atomic for allocating and
freeing.

Any better idea?

> .
>