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

On Tue, 22 Jun 2021 19:48:43 +0200 Eric Dumazet wrote:

quoted

quoted

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.

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.