cgroup: Use kernel command line to disable memory cgroup

[mairacanal]

Commit 94a23e9 ("cgroup: Disable cgroup "memory" by default") disabled the memory cgroup by default when initing the cgroups. However, it's possible to disable the memory cgroup by a kernel command line. Hard-coding such a feature can be problematic as some memory management features depend on the order that things are set.

For example, it is possible to see a NULL pointer dereference caused by commit 94a23e9. The NULL pointer dereference is triggered by the memory shrinker and ends up in a kernel crash.

[ 50.028629] ==================================================================
[ 50.028645] BUG: KASAN: null-ptr-deref in do_shrink_slab+0x1fc/0x978
[ 50.028663] Write of size 8 at addr 0000000000000000 by task gfxrecon-replay/1965

[ 50.028676] CPU: 3 UID: 1000 PID: 1965 Comm: gfxrecon-replay Tainted: G C 6.12.0-rc4-v8-thp-kasan+ #85
[ 50.028685] Tainted: [C]=CRAP
[ 50.028689] Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
[ 50.028694] Call trace:
[ 50.028697] dump_backtrace+0xfc/0x120
[ 50.028706] show_stack+0x24/0x38
[ 50.028711] dump_stack_lvl+0x40/0x88
[ 50.028720] print_report+0xe4/0x708
[ 50.028728] kasan_report+0xcc/0x130
[ 50.028733] kasan_check_range+0x254/0x298
[ 50.028738] __kasan_check_write+0x20/0x30
[ 50.028745] do_shrink_slab+0x1fc/0x978
[ 50.028751] shrink_slab+0x318/0xc38
[ 50.028756] shrink_one+0x254/0x6d8
[ 50.028762] shrink_node+0x26b4/0x2848
[ 50.028767] do_try_to_free_pages+0x3e4/0x1190
[ 50.028773] try_to_free_pages+0x5a4/0xb40
[ 50.028778] __alloc_pages_direct_reclaim+0x144/0x298
[ 50.028787] __alloc_pages_slowpath+0x5c4/0xc70
[ 50.028793] __alloc_pages_noprof+0x4a8/0x6a8
[ 50.028800] __folio_alloc_noprof+0x24/0xa8
[ 50.028806] shmem_alloc_and_add_folio+0x2ec/0xce0
[ 50.028812] shmem_get_folio_gfp+0x380/0xc20
[ 50.028818] shmem_read_folio_gfp+0xe0/0x160
[ 50.028824] drm_gem_get_pages+0x238/0x620 [drm]
[ 50.029039] drm_gem_shmem_get_pages_sgt+0xd8/0x4b8 [drm_shmem_helper]
[ 50.029053] v3d_bo_create_finish+0x58/0x1e0 [v3d]
[ 50.029083] v3d_create_bo_ioctl+0xac/0x210 [v3d]
[ 50.029105] drm_ioctl_kernel+0x1d8/0x2b8 [drm]
[ 50.029220] drm_ioctl+0x4b4/0x920 [drm]
[ 50.029330] __arm64_sys_ioctl+0x11c/0x160
[ 50.029337] invoke_syscall+0x88/0x268
[ 50.029345] el0_svc_common+0x160/0x1d8
[ 50.029351] do_el0_svc+0x50/0x68
[ 50.029358] el0_svc+0x34/0x80
[ 50.029364] el0t_64_sync_handler+0x84/0x100
[ 50.029371] el0t_64_sync+0x190/0x198
[ 50.029376] ==================================================================

This happens because the memory shrinker is unaware that we are artificially disabling the memory cgroups and therefore it doesn't allocate nr_deferred (as it would if we used the kernel command line).

To avoid such an issue, revert the artificial disablement and disable it through the command line. If a user wants to enable the feature, it can use the cgroup_enable= command line.

[pelwell]

Hi Maira,

Thanks for this. Your patches missed out a few platforms because of the non-trivial include hierarchy. You may find it easier to build on top of #6442, which is effectively a cosmetic change that reduces the number of files to change to 6:

arch/arm/boot/dts/broadcom/bcm2708-rpi-bt.dtsi:27:              bootargs = "coherent_pool=1M 8250.nr_uarts=1 snd_bcm2835.enable_headphones=0";
arch/arm/boot/dts/broadcom/bcm270x.dtsi:7:              bootargs = "coherent_pool=1M snd_bcm2835.enable_headphones=0";
arch/arm/boot/dts/broadcom/bcm2711-rpi-cm4s.dts:151:            bootargs = "coherent_pool=1M snd_bcm2835.enable_headphones=0 numa_policy=interleave";
arch/arm/boot/dts/broadcom/bcm2711-rpi-ds.dtsi:6:               bootargs = "coherent_pool=1M 8250.nr_uarts=1 snd_bcm2835.enable_headphones=0 numa_policy=interleave";
arch/arm/boot/dts/broadcom/bcm271x-rpi-bt.dtsi:27:              bootargs = "coherent_pool=1M 8250.nr_uarts=1 snd_bcm2835.enable_headphones=0";
arch/arm64/boot/dts/broadcom/bcm2712-rpi.dtsi:102:              bootargs = "reboot=w coherent_pool=1M 8250.nr_uarts=1 pci=pcie_bus_safe numa_policy=interleave iommu_dm

[mairacanal]

Thanks @pelwell! I'll wait #6442 to land and then I'll rebase this PR modifying the DTS needed.

[pelwell]

#6442 is merged, so over to you.

[mairacanal]

I rebased and updated the DTS files.

[pelwell]

Thanks!