Re: [PATCH net-next] ip: avoid OOM kills with large UDP sends over loopback

[Eric Dumazet <eric.dumazet@gmail.com>]

On 6/22/21 8:09 PM, Jakub Kicinski wrote:
> On Tue, 22 Jun 2021 19:48:43 +0200 Eric Dumazet wrote:
>>>> What about using 	sock_alloc_send_pskb(... PAGE_ALLOC_COSTLY_ORDER)
>>>> (as we did in unix_dgram_sendmsg() for large packets), for SG enabled interfaces ?  
>>>
>>> PAGE_ALLOC_COSTLY_ORDER in itself is more of a problem than a solution.
>>> AFAIU the app sends messages primarily above the ~60kB mark, which is
>>> above COSTLY, and those do not trigger OOM kills. All OOM kills we see
>>> have order=3. Checking with Rik and Johannes W that's expected, OOM
>>> killer is only invoked for allocations <= COSTLY, larger ones will just
>>> return NULL and let us deal with it (e.g. by falling back).  
>>
>> I  really thought alloc_skb_with_frags() was already handling low-memory-conditions.
>>
>> (alloc_skb_with_frags() is called from sock_alloc_send_pskb())
>>
>> If it is not, lets fix it, because af_unix sockets will have the same issue ?
> 
> af_unix seems to cap at SKB_MAX_ALLOC which is order 2, AFAICT.

It does not cap to SKB_MAX_ALLOC.

It definitely attempt big allocations if you send 64KB datagrams.

Please look at commit d14b56f508ad70eca3e659545aab3c45200f258c
    net: cleanup gfp mask in alloc_skb_with_frags

This explains why we do not have __GFP_NORETRY there.

> 
> Perhaps that's a good enough fix in practice given we see OOMs with
> order=3 only?
> 
> I'll review callers of alloc_skb_with_frags() and see if they depend 
> on the explicit geometry of the skb or we can safely fallback to pages.
>