Re: [PATCH net-next] mlx4: support __GFP_MEMALLOC for rx

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 18.01.2017 17:23, Eric Dumazet wrote:
> On Wed, 2017-01-18 at 12:31 +0300, Konstantin Khlebnikov wrote:
>> On 18.01.2017 07:14, Eric Dumazet wrote:
>>> From: Eric Dumazet <edumazet@google.com>
>>>
>>> Commit 04aeb56a1732 ("net/mlx4_en: allocate non 0-order pages for RX
>>> ring with __GFP_NOMEMALLOC") added code that appears to be not needed at
>>> that time, since mlx4 never used __GFP_MEMALLOC allocations anyway.
>>>
>>> As using memory reserves is a must in some situations (swap over NFS or
>>> iSCSI), this patch adds this flag.
>>
>> AFAIK __GFP_MEMALLOC is used for TX, not for RX: for allocations which
>> are required by memory reclaimer to free some pages.
>>
>> Allocation RX buffers with __GFP_MEMALLOC is a straight way to
>> depleting all reserves by flood from network.
>
> You are mistaken.
>
> How do you think a TCP flow can make progress sending data if no ACK
> packet can go back in RX ?

Well. Ok. I mistaken.

>
> Take a look at sk_filter_trim_cap(), where the RX packets received on a
> socket which does not have SOCK_MEMALLOC is dropped.
>
>         /*
>          * If the skb was allocated from pfmemalloc reserves, only
>          * allow SOCK_MEMALLOC sockets to use it as this socket is
>          * helping free memory
>          */
>         if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
>                 return -ENOMEM;

I suppose this happens in BH context right after receiving packet?

Potentially any ACK could free memory in TCP send queue,
so using reserves here makes sense.

>
> Also take a look at __dev_alloc_pages()
>
> static inline struct page *__dev_alloc_pages(gfp_t gfp_mask,
>                                              unsigned int order)
> {
>         /* This piece of code contains several assumptions.
>          * 1.  This is for device Rx, therefor a cold page is preferred.
>          * 2.  The expectation is the user wants a compound page.
>          * 3.  If requesting a order 0 page it will not be compound
>          *     due to the check to see if order has a value in prep_new_page
>          * 4.  __GFP_MEMALLOC is ignored if __GFP_NOMEMALLOC is set due to
>          *     code in gfp_to_alloc_flags that should be enforcing this.
>          */
>         gfp_mask |= __GFP_COLD | __GFP_COMP | __GFP_MEMALLOC;
>
>         return alloc_pages_node(NUMA_NO_NODE, gfp_mask, order);
> }
>
>
> So __GFP_MEMALLOC in RX is absolutely supported.
>
> But drivers have to opt-in, either using __dev_alloc_pages() or
> dev_alloc_pages, or explicitely ORing __GFP_MEMALLOC when using
> alloc_page[s]()
>
>
>


-- 
Konstantin

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