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 <redacted>
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#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.