netdev - Re: [PATCH] net: Use SK_MEM_QUANTUM as minimum for tcp/udp rmem/wmem

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Date:	Wed, 12 Aug 2015 07:21:01 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Calvin Owens <calvinowens@...com>,
	Sorin Dumitru <sdumitru@...acom.com>
Cc:	David Miller <davem@...emloft.net>, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com, sorin@...urnze.ro
Subject: Re: [PATCH] net: Use SK_MEM_QUANTUM as minimum for tcp/udp rmem/wmem

On Tue, 2015-08-11 at 21:54 -0700, Calvin Owens wrote:
> Commit 8133534c760d4083 ("net: limit tcp/udp rmem/wmem to
> SOCK_{RCV,SND}BUF_MIN") modified four sysctls to enforce that the values
> written to them are not less than SOCK_MIN_{RCV,SND}BUF.
> 
> That change causes 4096 (or SK_MEM_QUANTUM) to no longer be accepted as
> a valid value for 'min' in tcp_wmem and udp_wmem_min. 4096 has been the
> default for both of those sysctls for a long time, and unfortunately
> seems to be an extremely popular setting. This change breaks a large
> number of sysctl configurations at FB.
> 
> That commit referred to b1cb59cf2efe7971 ("net: sysctl_net_core: check
> SNDBUF and RCVBUF for min length"), which choose to use the SOCK_MIN
> constants as the lower limits to avoid nasty bugs. But AFAICS, a limit
> of SOCK_MIN_SNDBUF isn't necessary to do that: the BUG_ON cited in the
> commit message seems to have happened because unix_stream_sendmsg()
> expects a minimum of a full page (ie SK_MEM_QUANTUM) and the math broke,
> not because it had less than SOCK_MIN_SNDBUF allocated.
> 
> Nothing seems to assume that it has at least SOCK_MIN_SNDBUF to play
> with, so I think enforcing a minimum of SK_MEM_QUANTUM avoids the sort
> of bugs 8133534c was trying to avoid, and it does so without breaking
> anybody's sysctl configurations.
> 
> Fixes: 8133534c760d4083 ("net: limit tcp/udp rmem/wmem to SOCK_MIN...")
> Signed-off-by: Calvin Owens <calvinowens@...com>
> ---

#define SK_MEM_QUANTUM ((int)PAGE_SIZE)

Some arches have PAGE_SIZE = 65536

So your patch might break scripts as well for them.

We should revert 8133534c760d4083.


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