netdev - Re: [PATCH net-next] mlx4: support __GFP

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Date:   Wed, 18 Jan 2017 06:23:48 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH net-next] mlx4: support __GFP_MEMALLOC for rx

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@...gle.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 ?

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;

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]()