Re: [PATCH v6 1/4] mm/slub: enable debugging memory wasting of kmalloc

[Feng Tang]

Hi John,

Thanks for the bisecting and reporting!

On Mon, Oct 31, 2022 at 05:30:24AM +0800, Vlastimil Babka wrote:
> On 10/30/22 20:23, John Thomson wrote:
> > On Tue, 13 Sep 2022, at 06:54, Feng Tang wrote:
> >> kmalloc's API family is critical for mm, with one nature that it will
> >> round up the request size to a fixed one (mostly power of 2). Say
> >> when user requests memory for '2^n + 1' bytes, actually 2^(n+1) bytes
> >> could be allocated, so in worst case, there is around 50% memory
> >> space waste.
> > 
> > 
> > I have a ralink mt7621 router running Openwrt, using the mips ZBOOT kernel, and appear to have bisected
> > a very-nearly-clean kernel v6.1rc-2 boot issue to this commit.
> > I have 3 commits atop 6.1-rc2: fix a ZBOOT compile error, use the Openwrt LZMA options,
> > and enable DEBUG_ZBOOT for my platform. I am compiling my kernel within the Openwrt build system.
> > No guarantees this is not due to something I am doing wrong, but any insight would be greatly appreciated.
> > 
> > 
> > On UART, No indication of the (once extracted) kernel booting:
> > 
> > transfer started ......................................... transfer ok, time=2.01s
> > setting up elf image... OK
> > jumping to kernel code
> > zimage at:     80BA4100 810D4720
> > Uncompressing Linux at load address 80001000
> > Copy device tree to address  80B96EE0
> > Now, booting the kernel...
> 
> It's weird that the commit would cause no output so early, SLUB code is 
> run only later.
 
I noticed your cmdline has console setting, could you enable the
earlyprintk in cmdline like "earlyprintk=ttyS0,115200" etc to see
if there is more message printed out.

Also I want to confirm this is a boot failure and not only a boot
message missing.

> > Nothing follows
> > 
> > 6edf2576a6cc  ("mm/slub: enable debugging memory wasting of kmalloc") reverted, normal boot:
> > transfer started ......................................... transfer ok, time=2.01s
> > setting up elf image... OK
> > jumping to kernel code
> > zimage at:     80BA4100 810D47A4
> > Uncompressing Linux at load address 80001000
> > Copy device tree to address  80B96EE0
> > Now, booting the kernel...
> > 
> > [    0.000000] Linux version 6.1.0-rc2 (john@john) (mipsel-openwrt-linux-musl-gcc (OpenWrt GCC 11.3.0 r19724+16-1521d5f453) 11.3.0, GNU ld (GNU Binutils) 2.37) #0 SMP Fri Oct 28 03:48:10 2022
> > [    0.000000] SoC Type: MediaTek MT7621 ver:1 eco:3
> > [    0.000000] printk: bootconsole [early0] enabled
> > [    0.000000] CPU0 revision is: 0001992f (MIPS 1004Kc)
> > [    0.000000] MIPS: machine is MikroTik RouterBOARD 760iGS
> > [    0.000000] Initrd not found or empty - disabling initrd
> > [    0.000000] VPE topology {2,2} total 4
> > [    0.000000] Primary instruction cache 32kB, VIPT, 4-way, linesize 32 bytes.
> > [    0.000000] Primary data cache 32kB, 4-way, PIPT, no aliases, linesize 32 bytes
> > [    0.000000] MIPS secondary cache 256kB, 8-way, linesize 32 bytes.
> > [    0.000000] Zone ranges:
> > [    0.000000]   Normal   [mem 0x0000000000000000-0x000000000fffffff]
> > [    0.000000]   HighMem  empty
> > [    0.000000] Movable zone start for each node
> > [    0.000000] Early memory node ranges
> > [    0.000000]   node   0: [mem 0x0000000000000000-0x000000000fffffff]
> > [    0.000000] Initmem setup node 0 [mem 0x0000000000000000-0x000000000fffffff]
> > [    0.000000] percpu: Embedded 11 pages/cpu s16064 r8192 d20800 u45056
> > [    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 64960
> > [    0.000000] Kernel command line: console=ttyS0,115200 rootfstype=squashfs,jffs2
> > [    0.000000] Dentry cache hash table entries: 32768 (order: 5, 131072 bytes, linear)
> > [    0.000000] Inode-cache hash table entries: 16384 (order: 4, 65536 bytes, linear)
> > [    0.000000] Writing ErrCtl register=00019146
> > [    0.000000] Readback ErrCtl register=00019146
> > [    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
> > [    0.000000] Memory: 246220K/262144K available (7455K kernel code, 628K rwdata, 1308K rodata, 3524K init, 245K bss, 15924K reserved, 0K cma-reserved, 0K highmem)
> > [    0.000000] SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=4, Nodes=1
> > [    0.000000] rcu: Hierarchical RCU implementation.
> > 
> > 
> > boot continues as expected
> > 
> > 
> > possibly relevant config options:
> > grep -E '(SLUB|SLAB)' .config
> > # SLAB allocator options
> > # CONFIG_SLAB is not set
> > CONFIG_SLUB=y
> > CONFIG_SLAB_MERGE_DEFAULT=y
> > # CONFIG_SLAB_FREELIST_RANDOM is not set
> > # CONFIG_SLAB_FREELIST_HARDENED is not set
> > # CONFIG_SLUB_STATS is not set
> > CONFIG_SLUB_CPU_PARTIAL=y
> > # end of SLAB allocator options
> > # CONFIG_SLUB_DEBUG is not set
> 
> Also not having CONFIG_SLUB_DEBUG enabled means most of the code the 
> patch/commit touches is not even active.
> Could this be some miscompile or code layout change exposing some 
> different bug, hmm.
> Is it any different if you do enable CONFIG_SLUB_DEBUG ?
> Or change to CONFIG_SLAB? (that would be really weird if not)

I haven't found any clue from the code either, and I compiled
kernel with the config above and tested booting on an Alder-lake
desktop and a QEMU, which boot fine.

Could you provide the full kernel config and demsg (in compressed
format if you think it's too big), so we can check more?

Thanks,
Feng