Re: [PATCH net-next v6 0/8] sysctl: Remove sentinel elements from networking

Sabrina Dubroca
2024-05-01T13:22:20Z

2024-05-01, 11:29:24 +0200, Joel Granados via B4 Relay wrote:
> From: Joel Granados <j.granados@samsung.com>
>
> What?
> These commits remove the sentinel element (last empty element) from the
> sysctl arrays of all the files under the "net/" directory that register
> a sysctl array. The merging of the preparation patches [4] to mainline
> allows us to just remove sentinel elements without changing behavior.
> This is safe because the sysctl registration code (register_sysctl() and
> friends) use the array size in addition to checking for a sentinel [1].
>
> Why?
> By removing the sysctl sentinel elements we avoid kernel bloat as
> ctl_table arrays get moved out of kernel/sysctl.c into their own
> respective subsystems. This move was started long ago to avoid merge
> conflicts; the sentinel removal bit came after Mathew Wilcox suggested
> it to avoid bloating the kernel by one element as arrays moved out. This
> patchset will reduce the overall build time size of the kernel and run
> time memory bloat by about ~64 bytes per declared ctl_table array (more
> info here [5]).
>
> When are we done?
> There are 4 patchest (25 commits [2]) that are still outstanding to
> completely remove the sentinels: files under "net/" (this patchset),
> files under "kernel/" dir, misc dirs (files under mm/ security/ and
> others) and the final set that removes the unneeded check for ->procname
> == NULL.
>
> Testing:
> * Ran sysctl selftests (./tools/testing/selftests/sysctl/sysctl.sh)
> * Ran this through 0-day with no errors or warnings
>
> Savings in vmlinux:
> A total of 64 bytes per sentinel is saved after removal; I measured in
> x86_64 to give an idea of the aggregated savings. The actual savings
> will depend on individual kernel configuration.
> * bloat-o-meter
> - The "yesall" config saves 3976 bytes (bloat-o-meter output [6])
> - A reduced config [3] saves 1263 bytes (bloat-o-meter output [7])
>
> Savings in allocated memory:
> None in this set but will occur when the superfluous allocations are
> removed from proc_sysctl.c. I include it here for context. The
> estimated savings during boot for config [3] are 6272 bytes. See [8]
> for how to measure it.
>
> Comments/feedback greatly appreciated
>
> Changes in v6:
> - Rebased onto net-next/main.
> - Besides re-running my cocci scripts, I ran a new find script [9].
> Found 0 hits in net/
> - Moved "i" variable declaraction out of for() in sysctl_core_net_init
> - Removed forgotten sentinel in mpls_table
> - Removed CONFIG_AX25_DAMA_SLAVE guard from net/ax25/ax25_ds_timer.c. It
> is not needed because that file is compiled only when
> CONFIG_AX25_DAMA_SLAVE is set.
> - When traversing smc_table, stop on ARRAY_SIZE instead of ARRAY_SIZE-1.
> - Link to v5: https://lore.kernel.org/r/20240426-jag-sysctl_remset_net-v5-0-e3b12f6111a6@samsung.com

I pointed out a few tiny details in the ax25 patch but either way, the
series looks good to me. Thanks!

Series:
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>

Note that you could have kept the ack/reviewed-by on patch 4 since it
was not modified. Jeff and Chuck, your reviews got lost in the repost.

--
Sabrina