netdev - Re: [PATCH 1/2 net-next] net: Don't return pfmemalloc pages to the page pool.

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Date:   Mon, 07 Jan 2019 11:10:01 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Jesper Dangaard Brouer" <brouer@...hat.com>
Cc:     netdev@...r.kernel.org,
        "Alexander Duyck" <alexander.duyck@...il.com>,
        "Alexei Starovoitov" <alexei.starovoitov@...il.com>,
        kernel-team@...com, "Jes Sorensen" <jsorensen@...com>,
        "Saeed Mahameed" <saeedm@...lanox.com>,
        "David Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/2 net-next] net: Don't return pfmemalloc pages to the
 page pool.

On 5 Jan 2019, at 7:46, Jesper Dangaard Brouer wrote:

> On Thu, 20 Dec 2018 14:21:32 -0800
> Jonathan Lemon <jonathan.lemon@...il.com> wrote:
>
>> Return pfmemalloc pages back to the page allocator, instead of 
>> holding them
>> in the page pool.
>>
>> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
>> ---
>>  net/core/page_pool.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>> index 43a932cb609b..364b893be66f 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -233,7 +233,7 @@ void __page_pool_put_page(struct page_pool *pool,
>>  	 *
>>  	 * refcnt == 1 means page_pool owns page, and can recycle it.
>>  	 */
>> -	if (likely(page_ref_count(page) == 1)) {
>> +	if (likely(page_ref_count(page) == 1 && !page_is_pfmemalloc(page))) 
>> {
>
> I took at closer look at the page_pool issue recycling pages from
> emergency reserve (pfmemalloc), and it actually cannot happen, because
> page_pool does not use the __GFP_MEMALLOC gfp_t flag. Thus, page_pool
> are not allowed to get pages from the emergency reserve in the first
> place (unless ksoftirqd current->flags have PF_MEMALLOC, which I don't
> think it have).

page_pool_dev_alloc_pages() sets GFP_ATOMIC | __GFP_NOWARN.  If the
page pool really doesn't want to allow emergency reserves, then 
shouldn't
it set __GFP_NOMEMALLOC as well?



> See: page_pool_dev_alloc_pages() compared to __dev_alloc_pages().
>
> The doc for:
> /* %__GFP_MEMALLOC allows access to all memory. This should only be 
> used when
>  * the caller guarantees the allocation will allow more memory to be 
> freed
>  * very shortly e.g. process exiting or swapping. Users either should
>  * be the MM or co-ordinating closely with the VM (e.g. swap over 
> NFS).
>  */
>
> With that desc, I don't understand why we actually allow 
> dev_alloc_pages()
> to get emergency reserve (pfmemalloc) pages, as we store these in an
> RX-ring queue (usual size 512-1024) that isn't used until N-packets
> later... even if used as a signal to network stack, to free other
> resources, this happens at a later point-in-time, not "very shortly".
>
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer