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@xxxxxxxxxx>
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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