Reading `/sys/kernel/debug/kmemleak` takes 3 s and content not shown

Tue Mar 26 23:49:33 AEDT 2019

Dear Linux folks,

On 19.02.19 10:44, Paul Menzel wrote:

> On a the IBM S822LC (8335-GTA) with Ubuntu 18.10, and Linux 5.0-rc5+
> accessing `/sys/kernel/debug/kmemleak` takes a long time. According to
> strace it takes three seconds.
> 
> ```
> $ sudo strace -tt -T cat /sys/kernel/debug/kmemleak
> 10:35:49.861641 execve("/bin/cat", ["cat", "/sys/kernel/debug/kmemleak"], 0x7ffffdbcb518 /* 16 vars */) = 0 <0.000293>
> 10:35:49.862112 brk(NULL)               = 0x75b12a50000 <0.000012>
> 10:35:49.862190 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) <0.000015>
> 10:35:49.862261 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory) <0.000015>
> 10:35:49.862324 openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 <0.000018>
> 10:35:49.862389 fstat(3, {st_mode=S_IFREG|0644, st_size=143482, ...}) = 0 <0.000011>
> 10:35:49.862444 mmap(NULL, 143482, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ce4a1150000 <0.000017>
> 10:35:49.862501 close(3)                = 0 <0.000011>
> 10:35:49.862550 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) <0.000015>
> 10:35:49.862615 openat(AT_FDCWD, "/lib/powerpc64le-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 <0.000019>
> 10:35:49.862676 read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0\25\0\1\0\0\0pN\2\0\0\0\0\0"..., 832) = 832 <0.000011>
> 10:35:49.862731 fstat(3, {st_mode=S_IFREG|0755, st_size=2310856, ...}) = 0 <0.000011>
> 10:35:49.862783 mmap(NULL, 2380672, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ce4a0f00000 <0.000018>
> 10:35:49.862842 mprotect(0x7ce4a1120000, 65536, PROT_NONE) = 0 <0.000019>
> 10:35:49.862899 mmap(0x7ce4a1130000, 131072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x220000) = 0x7ce4a1130000 <0.000019>
> 10:35:49.862990 close(3)                = 0 <0.000010>
> 10:35:49.863110 mprotect(0x7ce4a1130000, 65536, PROT_READ) = 0 <0.000017>
> 10:35:49.863192 mprotect(0x75ad43b0000, 65536, PROT_READ) = 0 <0.000016>
> 10:35:49.863252 mprotect(0x7ce4a11e0000, 65536, PROT_READ) = 0 <0.000015>
> 10:35:49.863305 munmap(0x7ce4a1150000, 143482) = 0 <0.000022>
> 10:35:49.863446 brk(NULL)               = 0x75b12a50000 <0.000011>
> 10:35:49.863495 brk(0x75b12a80000)      = 0x75b12a80000 <0.000014>
> 10:35:49.863561 openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3 <0.000019>
> 10:35:49.863624 fstat(3, {st_mode=S_IFREG|0644, st_size=6035920, ...}) = 0 <0.000010>
> 10:35:49.863677 mmap(NULL, 6035920, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ce4a0930000 <0.000017>
> 10:35:49.863736 close(3)                = 0 <0.000011>
> 10:35:49.863828 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0 <0.000010>
> 10:35:49.863881 openat(AT_FDCWD, "/sys/kernel/debug/kmemleak", O_RDONLY) = 3 <0.000034>
> 10:35:49.863956 fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 <0.000029>
> 10:35:49.864028 fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0 <0.000011>
> 10:35:49.864076 mmap(NULL, 262144, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ce4a08f0000 <0.000017>
> 10:35:49.864146 read(3, "", 131072)     = 0 <3.528503>
> 10:35:53.392797 munmap(0x7ce4a08f0000, 262144) = 0 <0.000092>
> 10:35:53.392957 close(3)                = 0 <0.000029>
> 10:35:53.393038 close(1)                = 0 <0.000010>
> 10:35:53.393078 close(2)                = 0 <0.000009>
> 10:35:53.393123 exit_group(0)           = ?
> 10:35:53.393280 +++ exited with 0 +++
> $ uname -a
> Linux flughafenberlinbrandenburgwillybrandt 5.0.0-rc5+ #1 SMP Thu Feb 7 11:23:11 CET 2019 ppc64le ppc64le ppc64le GNU/Linux
> $ more /proc/version
> Linux version 5.0.0-rc5+ (pmenzel at flughafenberlinbrandenburgwillybrandt) (gcc version 8.2.0 (Ubuntu 8.2.0-7ubuntu1)) #1 SMP Thu Feb 7 11:23:11 CET 2019
> $ more /proc/cmdline
> root=UUID=2c3dd738-785a-469b-843e-9f0ba8b47b0d ro rootflags=subvol=@ quiet splash
> $ grep KMEMLEAK /boot/config-5.0.0-rc5+
> CONFIG_HAVE_DEBUG_KMEMLEAK=y
> CONFIG_DEBUG_KMEMLEAK=y
> CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE=10000
> # CONFIG_DEBUG_KMEMLEAK_TEST is not set
> # CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set
> CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN=y
> $ grep KMEMLEAK /boot/config-4.18.0-rc4+
> CONFIG_HAVE_DEBUG_KMEMLEAK=y
> CONFIG_DEBUG_KMEMLEAK=y
> CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE=10000
> # CONFIG_DEBUG_KMEMLEAK_TEST is not set
> # CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set
> $ dmesg | grep leak
> [    4.407957] kmemleak: Kernel memory leak detector initialized
> [    4.407959] kmemleak: Automatic memory scanning thread started
> [745989.625624] kmemleak: 1 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
> [1002619.951902] kmemleak: 1 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
> ```
> 
> Unfortunately, the leaks supposedly stored in that file are not shown
> either.

The problem is still present with Linux 5.0. Do you have an idea, how to 
fix this?

Kind regards,

Paul