Harry (Hyeonggon) Yoo
Posts
491Following
97Followers
113- Linux Kernel Developer @ Oracle (Linux Kernel MM) (2025.02 ~ Present)
- Reviewer for the Linux Slab subsystem
- Former Intern @ NVIDIA, SK Hynix, Panmnesia (Security, MM and CXL)
- B.Sc. in Computer Science & Engineering, Chungnam National University (Class of 2025)
Opinions are my own.
My interests are:
Memory Management,
Computer Architecture,
Circuit Design,
Virtualization
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
it is interesting to learn how PCIe endpoints and switches implements their own protocols (similar to network protocols) for communication (PCIe transport/data link/physical layers)
but since I know very little about electronics some topics are hard to understand, like why it's difficult to increase speed in parallel bus architecture or why errors occur during transmission, or why DC components should be avoided.
but since I know very little about electronics some topics are hard to understand, like why it's difficult to increase speed in parallel bus architecture or why errors occur during transmission, or why DC components should be avoided.
3
0
0
Harry (Hyeonggon) Yoo
repeated
repeated
LWN.net is now @LWN@lwn.net
LWN@fosstodon.org[$] Progress on no-GIL CPython https://lwn.net/Articles/947138/ #LWN
0
2
1
Harry (Hyeonggon) Yoo
hyeyoo
until yesterday I didn't know that my laptop has 2 NUMA nodes, but why?
2
0
3
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
Learning how to write a LAVA test definition, but the tricker thing is to decide which tests to run to verify a kernel works fine.
Candidates:
- LTP
- KUnit
- kselftests
btw it is funny that the entire LTP suite gets killed every time it runs oom testcases. and LTP takes quite long time for a lightweight testing.
hmm... maybe run only a smaller subset of them?
Candidates:
- LTP
- KUnit
- kselftests
btw it is funny that the entire LTP suite gets killed every time it runs oom testcases. and LTP takes quite long time for a lightweight testing.
hmm... maybe run only a smaller subset of them?
0
0
1
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
Today I learned:
CXL memory can be mapped as 'System RAM' or 'Soft Reserved' by platform firmware. Or it can be dynamically provisioned by (since v6.3) CXL region driver.
And 'Soft Reserved' or dynamically provisioned CXL RAM region can be used in two ways:
1. Applications mmap() to /dev/daxX.Y files, just like traditional persistent memory devices.
2. Kernel use it as System RAM via dax_kmem driver.
And a weird fact is that when dax_kmem onlines CXL (and other performance-differentiated like pmem) memory, to ZONE_NORMAL, not ZONE_MOVABLE.
CXL memory can be mapped as 'System RAM' or 'Soft Reserved' by platform firmware. Or it can be dynamically provisioned by (since v6.3) CXL region driver.
And 'Soft Reserved' or dynamically provisioned CXL RAM region can be used in two ways:
1. Applications mmap() to /dev/daxX.Y files, just like traditional persistent memory devices.
2. Kernel use it as System RAM via dax_kmem driver.
And a weird fact is that when dax_kmem onlines CXL (and other performance-differentiated like pmem) memory, to ZONE_NORMAL, not ZONE_MOVABLE.
2
0
1
Harry (Hyeonggon) Yoo
repeated
repeated
Working on a series for 6.8 probably...
So far...
26 files changed, 39 insertions(+), 4375 deletions(-)
So far...
26 files changed, 39 insertions(+), 4375 deletions(-)
3
2
16
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
Booting the latest kernel is always fun ;)
I was hit by BUG_ON() because I enabled CONFIG_DEBUG_VIRTUAL=y.
It made my machine crash and I wrote and sent a quick hack to fix this.
https://lore.kernel.org/lkml/CAB=+i9QiJ=BXkQuCFJTh3dMXrkKQvVA2EM51Mj6SsDMimWQ71g@mail.gmail.com
Today I learned:
- Better to use proper tags for regzbot next time I report something, don't make @kernellogger do that instead of me.
- __init and __exit are compiler attributes that makes functions to be in specific ELF sections (.init.text and .exit.text), to drop portions of unused kernel code.
- When a kernel feature is built into kernel rather than built as a module, functions marked __init are dropped after initialization and functions marked __exit are dropped and never used. because you can't unload a built-in kernel feature ;)
- Some architectures drop .exit.text section at link time, but some drop at runtime. this is due to complexity of link-time reference counting between functions (? which I have no idea yet)
- On architectures that drop it at runtime, functions marked __exit are dropped in free_initmem() because .exit.text section is between __init_begin and __init_end.
- I need an automatic bisection system to save time in the future.
One piece of information I'm missing here is why it did not crash before the commit :(
I was hit by BUG_ON() because I enabled CONFIG_DEBUG_VIRTUAL=y.
It made my machine crash and I wrote and sent a quick hack to fix this.
https://lore.kernel.org/lkml/CAB=+i9QiJ=BXkQuCFJTh3dMXrkKQvVA2EM51Mj6SsDMimWQ71g@mail.gmail.com
Today I learned:
- Better to use proper tags for regzbot next time I report something, don't make @kernellogger do that instead of me.
- __init and __exit are compiler attributes that makes functions to be in specific ELF sections (.init.text and .exit.text), to drop portions of unused kernel code.
- When a kernel feature is built into kernel rather than built as a module, functions marked __init are dropped after initialization and functions marked __exit are dropped and never used. because you can't unload a built-in kernel feature ;)
- Some architectures drop .exit.text section at link time, but some drop at runtime. this is due to complexity of link-time reference counting between functions (? which I have no idea yet)
- On architectures that drop it at runtime, functions marked __exit are dropped in free_initmem() because .exit.text section is between __init_begin and __init_end.
- I need an automatic bisection system to save time in the future.
One piece of information I'm missing here is why it did not crash before the commit :(
2
0
4
Harry (Hyeonggon) Yoo
repeated
repeated
Thorsten Leemhuis (acct. 1/4)
kernellogger@fosstodon.org"[…] Instead of accepting my [#LinuxKernel] patch or guiding me towards a better solution, he went ahead and implemented his own fix, giving me credit only for reporting the issue […]
My first contribution to the [#Linux] #kernel was a really frustrating and discouraging experience, dealing with people who do not think it’s important to get proper recognition for your work. […]"
https://ariel-miculas.github.io/How-I-got-robbed-of-my-first-kernel-contribution/
1/ Side note: I sometimes wonder if…
2
3
1
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
My android phone kills the chrome app very often. It is painful that I can't see system logs. I suspect it was the OOM killer that kills it.
2
0
4
Harry (Hyeonggon) Yoo
repeated
repeated
Liam @ GamingOnLinux 🐧🎮
gamingonlinux@mastodon.socialRaspberry Pi 5 announced - still tiny, much more powerful https://www.gamingonlinux.com/2023/09/raspberry-pi-5-announced-still-tiny-much-more-powerful/
0
2
1
Harry (Hyeonggon) Yoo
repeated
repeated
Dare Obasanjo
carnage4life@mas.toWelcome to the cycle of generative AI making search worse.
Quora uses ChatGPT which hallucinates an answer to a nonsense question.
Google Search picks up this nonsense answer from Quora which has high page rank and treats it as an instant answer.
9
11
3
Harry (Hyeonggon) Yoo
repeated
repeated
K. Ryabitsev 🍁
monsieuricon
"Subscribing" to mailing lists via sourcing a pop3 mailbox from lore.kernel.org now works. Should be of particular interest to gmail users.
I'll document after some more initial testing.
I'll document after some more initial testing.
1
5
12
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
small progress on the kernel testing side project: I did setup LAVA instance in addition to Jenkins and made it publicly available.
it took few weeks due to issues with hardware :(
this is home server so can't provide excellent stability and uptime but here it is:
https://lava.kerneltesting.org
https://jenkins.kerneltesting.org
it took few weeks due to issues with hardware :(
this is home server so can't provide excellent stability and uptime but here it is:
https://lava.kerneltesting.org
https://jenkins.kerneltesting.org
2
0
2
Harry (Hyeonggon) Yoo
hyeyoo
my jenkins VM was mysteriously shut down continuously without any log. I spent some time figuring out why, but it was the host's OOM killer that killed it. chrome tabs that I forgot to close was eating up 100% of the memory. fun ;)
but 32gib for a browser was too much? was it memory leak?
but 32gib for a browser was too much? was it memory leak?
1
0
1
Harry (Hyeonggon) Yoo
repeated
repeated
Kees Cook (old account)
kees@fosstodon.org
Edited 1 year ago
I'm really excited about making Use-After-Free exploits much harder in the #Linux #kernel.
CONFIG_RANDOM_KMALLOC_CACHES has landed:
https://git.kernel.org/linus/3c6152940584
CONFIG_SLAB_VIRTUAL is coming:
https://lore.kernel.org/lkml/20230915105933.495735-15-matteorizzo@google.com/
2
10
3
Harry (Hyeonggon) Yoo
hyeyoo
writing code that will not be used at all is utterly frustrating and akin to creating garbage.
2
0
1
Harry (Hyeonggon) Yoo
hyeyoo
Edited 1 year ago
Buried in the work... feeling like being a piece of shit. Oh, I should visit this:
https://hypebeast.com/2023/9/joan-cornella-vip-solo-exhibition-allrightsreserved-meet-project-em-gallery-seoul
https://hypebeast.com/2023/9/joan-cornella-vip-solo-exhibition-allrightsreserved-meet-project-em-gallery-seoul
1
0
3
Harry (Hyeonggon) Yoo
repeated
repeated
Jonathan Corbet
corbet
It has happened at last: after many iterations, that x86 shadow-stack patch series has been accepted into the mainline for 6.6:
https://lwn.net/ml/linux-kernel/CAHk-=wipDVFEfSJxbnUSDqENs5i8RzSsgJOFdb69pH=b7BOFiQ@mail.gmail.com/
For those who haven't been following this saga, see https://lwn.net/Articles/926649/
https://lwn.net/ml/linux-kernel/CAHk-=wipDVFEfSJxbnUSDqENs5i8RzSsgJOFdb69pH=b7BOFiQ@mail.gmail.com/
For those who haven't been following this saga, see https://lwn.net/Articles/926649/
0
10
18